External monitor flickers

I just a AOC 24" monitor, it replaced a Samsung 19" monitor I had. But with this new monitor the screen flickers and the resolution looks awful. I changed the refresh rate to 60hz and I'm using a HDMI/ DVI adapter to connect my DVI cable which then connects to the mini display port then to MB.
Should I return the monitor and get something else? I'm trying to save money as I'm trying to start my own business.

I have a similar issues, but I never got to the bottom of the issue.
I have a BenQ LCD 22' monitor and also a Sharp 40' LCD TV.
If I connect the monitor via DVI using a MiniDisplayPort to DVI adaptor it connects flawlessly.
If I connect the monitor via HDMI using a MiniDisplayPort to HDMI adaptor I get the annoying flicker. Doesn't seem to matter which resolution I pick.
If I connect the TV via the same HDMI adaptor, it works flawlessly, So I don't think it the adaptor making me think that it is the monitor.
However, if I connect the monitor to a Digital Set Top box via HDMI, the monitor work flawlessly, so it appears the the monitor is OK too.
My current work around is to always connect the monitor via DVI and use the HDMI for the TV.
Maybe take the macbook to the store and test it on another monitor and see if the issue is specific to the monitor you have.

Similar Messages

External monitor flickers when waking up from sleep

I use an external monitor with my new MacBook. All works well, but I have noticed that if I start using the external after the screen has gone to sleep, I sometimes get a problem on the external where the screen flickers and breaks up. If I disconnect and reconnect the Mini DisplayPort then the problem resolves, has anyone else seen this?
Tony

I have the same issue with my new macbook.
When i wake the computer from a partial sleep the screen flickers and i get line distortion through the image.
If i put the machine to sleep unplug the mini dvi adapter and replug it in and wake the computer up the problem disappears.
The issue is also intermittent doing about 3 times out of every 10 wake ups...
If anyone has any suggestions it would be great, I a using a new Dell 2208WFP monitor.

External monitor flickers when scroll on the google stock chart in Safari

Well, this sounds crazy but this is what is happening to my Retina, 15-inch, Late 2013 Macbook Pro.
I can reproduce it.
1. Go to this page in Safari (Version 7.1 (9537.85.10.17.1)): https://www.google.com/finance?q=LQD&ei=nQQ_VPj1H4ebrAHP6oDYDQ
2. Scroll the page (not the chart)
3. My external monitor starts to flicker.
I use mini display port to DVI to connect to my ASUS VE278.
Any idea? Thanks in advance!

Hi Ziang Song,
Welcome to the Apple Support Community!
I know it can be concerning when your external display is flickering and not functioning as you would expect. In this situation I recommend the troubleshooting steps outlined in the attached article.
Apple computers: Troubleshooting issues with video on internal or external displays
Cheers,
Joe

Tecra M5: External monitor flickers - grounding issue?

Hi,
I just bought a new Acer Full HD LCD Monitor (P244W) and have tried connecting it to the VGA port of my Tecra M5.
It works, but there's a noticable and increasingly annoying flickering. I tried it with both XP (SP3) and now Vista (SP1) and it happens for both. I have the latest BIOS and NVidia drivers. The actual laptop LCD screen still works fine, its just the external monitor that flickers.
To ensure there was nothing wrong with the monitor, I connected it to the VGA port on the ATI video card on my desktop pc and it displayed fine without any flickering.
After browsing the web for people having similar problems, I've narrowed it down to it being caused by my AC Adapter having a grounding problem as the flickering dissapears as soon as I disconnect the AC Adapter and just run from the battery. It the seems the common solutons to this problem are:
1) Try some different VGA cables with better shielding from interference.
I tried this and it made no difference. I tried cables both with and without the ferrite magnet/choke thingy's at both ends of the cable and the flickering remained.
2) Try different screen resolutions and refresh rates.
Tried setting some lower resolutions, but the flickering remained (besides I'd rather keep the 1920x1080 native resolution for my monitor). The NVidia driver only allows 60Hz for the refresh rate at 1920x1080, so I couldn't change this.
3) Ensure both the laptop AC Adapter and the monitor are connected to the same wall socket and are on the same power extension board.
Tried this, but didn't fix it.
4) Run an extension cable to another power circuit in my house and power the AC Adapter and monitor from this to see if its something wrong with the wall socket im using.
Tried connecting to a wall socket in my laundry, but didn't fix it.
5) Try using a different power extension board and ensure nothing else is connected on it other than the AC Adapter and the monitor.
Tried this, but didn't fix it.
6) Get a 3 prong to 2 prong adapter, so the AC Adapter is no longer grounded.
Most people on the web seem to say this fixes the problem, but I'm not sure how safe it is to leave it this way in the long run. Especially, when stories of laptops catching on fire are becoming more common.
7) Get a generic brand AC Adapter as these sometimes deal with the grounding problem better than the Toshiba AC Adapter.
Again, I'd rather avoid this option as even the generic branded ones are expensive.
8) Remove the battery and run from the AC Adapter on its own.
Tried this, but didn't fix it.
9) Fiddle about with the monitors settings.
Tried this, but none of the settings fixed it.
So, what are my options?
Is there a known cause & solution to this problem?
If my laptop is still under warranty, can I get a replacement AC Adapter or something to workaround this problem?
Thanks
Message was edited by: shaneh20
Message was edited by: shaneh20

I finally figured it out. Just for laughs and because it was the only thing I hadn't tried, I tried using a different 3 prong power cable that I had lying around, connected it to my monitor and the flickering dissapeared. *So, the power cable that came with my Acer monitor was dodgy and not my Toshy's AC Adapter like I first thought.*
I'm guessing the dodgy power cable was preventing the monitor from grounding properly, so instead it tried to ground itself using my laptop's power cable, going via the VGA cable
. This would explain why the problem seemed to go away whenever I unplugged my laptop's power cable and also why there was interference.

External monitor flickering issues on T520

Hi, I have a Lenovo T520 with Nvidia NVS 4200M with the latest driver installed. I pluged both a VGA external monitor, then removed it and pluged a newer Displayport monitor and both monitors flicker. Thank youFrançois

Yes they are all on recommended resolutions. I did a factory reset on my Dell Ultrasharp monitor (with mini-displayport to displayport cable) but it did the same flickering on an older HP LCD display with VGA connexion. I'm going to try changing the vsync settings. It is on "use the 3D application setting" now but I will try "adaptive". And I also noticed there is a newer Nvidia driver from windows update, I will try that after. Thank you!Frankie

My external monitor flickers intermittently and wont function when connected to macbook pro

Hello everyone,
I recently purchased a cable to convert the thunderbolt/miniport to the VGA input on an external monitor I have had laying around for a few years. I am currently running osx 10.7.5 on a macbook pro.
When I plugged the cable to connect my macbook to the monitor, the monitor certainly recognized the cable, but the display would not do more than flicker in and out intermittently when connected to my macbook.
I have searched the forums for a solution to this, but haven't been able to find one that works. I tried resetting the SMC and restarting while holding cmd+P+R but none of these "fixes" seemed to help.

Simplest thing to try is a PRAM reset.
Shut down your Mac.
Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.
Excerpt from http://support.apple.com/kb/ht1379

External Monitor Flickers Slowly

I have an iMac 8,1 with a Dell 2009W connected via DVI at 1680x1050 and 60 Hz. Every couple mins the screen will go black for half a second then back to normal several times. I haven't seen anyone else on the forums with the same issue; they all seem to have constant, rapid flickering. Any ideas? I use the external to expand screen area for Pro Tools and it's affecting my work.

I have just moved and only have my iMac, Dell monitor, and printer connected. It is unrelated to audio equipment. not to mention I don't have any unshielded cables or equipment.

External monitor connected to new MacBook Pro

I have a Samsung LED monitor connected to my brand new MacBook Pro with a mini display port adapter and a DVI-D dual link digital male to male HDTV cable. The external monitor flickers, cuts in and out and has static similar to an analog TV that can't find a signal. I'm wondering if it's the wrong kind of cable? Any help would be appreciated.

I have the same thing but with a 23 inch HP, worked fine with my MBP 13 from 2009 but the 2011 2.4 i5 does the exact same thing as yours!
Very confused and hoping that some one else can help.
Thanks

External Disply flickers and went Black for a short duration after yosemite update 10.10.2

hi,
i got a mac book pro late 2008 and since todays update my external monitor flickers and went black for a short duration.
It seems that it happens, when i start a new app or i move a window across the displays.
Disabling the automated scaling of the resolution reduced the flickering but it still happens.
Does anyone faces the same problems and have an idea how to fix it?
I would appreciate it.
Sincerely
Lukas

Hello lukasfromholzwickede,
Thanks for participating in the Apple Support Communities.
It sounds like you've noticed flickering on your external display after updating OS X Yosemite.
I'd recommend starting with these resets to troubleshoot the situation:
You can reset the Mac's parameter RAM and SMC.
You can find these steps in this article:
Apple computers: Troubleshooting issues with video on internal or external displays - Apple Support
If the behavior continues, try going through the steps in the "Update your software and firmware" and "No video or no signal, image distortion, 'snow,' or flickering" sections of this article, as well.
All the best,
Jeremy

Flickering screen on external monitor

Hi, when I connect my 2011 Macbook Air to a external monitor, the screen of the external monitor is flickering. Tried different cables and monitors, same problem every time. So my guess is, that it's a problem with the hardware of my MB? Is it easy to fix, will Apple fix it when I'll bring it to the Apple Store? Actually, I can't live without my Macbook for a while, because of my work. The problem was always there, but it's getting worse and annoying, because I bought a new very expensive external monitor.

Same with my macbook pro 13" 7,1 (mid 2010) but also with resolution same or under 1080p

Flickering on DVI external monitor connected to 2009 Macbook

This is an ongoing an intermittent problem. At one point Apple phone support suggested that engineers are working on the problem but I haven't heard any updates.
The problem is with a 2009 macbook (13 inch) connected to an apple display (from 2008) by an apple minidisplayport to dvi connector. I don't find this kind of problem when another computer is connected to the display. The problem is unaffected by changing the brightness settings on the display.
The problem is intermittent, it occurs roughly once every five days. It typically occurs after the computer wakes from sleep. It doesn't usually occur when the macbook is first connected to the display (this suggests that it is not a problem with the minidisplayport to dvi cable) and jiggling the cable doesn't cause the problem or make it go away. The problem doesn't occur in particular when the computer is hot, it could be cool, after it is woken from sleep. There are no problems on the macbook's built in display.
The flickering is quite dramatic. Dark flashes extend from the bottom of the display sometimes 50% of the way up the screen and sometime 75% of the way up. The frequency is irregular but during an episode the flickers occur about once every one or two seconds, but sometimes there is a gap of 5 seconds. The episode will come to an end if I put the machine to sleep and wake it again (this also suggests that this is not a problem with the cable connection or the monitor).
It is difficult to get support on this issue. Since the problem is intermittent and may not occur for 5 days it doesn't make sense to try to reproduce it in an apple store. I called apple phone support and the technician recommended adjusting the brightness of the display, resetting the pram, moving some files that record configuration data for the external monitor. These didn't help. The technician acknowledged that apple is working on this problem and recommended that I call back if the problem recurred. I called back a couple weeks later and a technician recommended that I buy another minidisplayport dvi connector - to be frank this is an inconsiderate suggestion for a problem with end to end apple hardware - and I don't expect it to solve the problem.
How should I troubleshoot this problem, or get effective help from Apple? Is this a problem with the graphics card?
John

Hello, i have the same kind of issue.
75% of the time, my external screen works fine, otherwise, i need 5-10 plug/unplug/reboot to make it work...
There my windowserver.log from this morning :
Jul 28 09:40:00 [52] Server is starting up
Jul 28 09:40:01 [52] CGXMappedDisplayStart: Unit 0: boot screen
Jul 28 09:40:01 [52] CGXMappedDisplayStart: Unit 0: no display alias property
Jul 28 09:40:02 [52] CGXMappedDisplayStart: Unit 1: no display alias property
Jul 28 09:40:10 [52] AGCAttach: Couldn't find any matches
Jul 28 09:40:10 [52] CGXPerformInitialDisplayConfiguration
Jul 28 09:40:10 [52] Display 0x4272800: MappedDisplay Unit 0; Vendor 0x610 Model 0x9ca0 S/N 0; online enabled built-in (0,0)[1280 x 800], base addr 0x1800000
Jul 28 09:40:10 [52] Display 0x3f003d: MappedDisplay Unit 1; Vendor 0xffffffff Model 0xffffffff S/N -1; offline enabled (2304,0)[1 x 1], base addr 0x1800000
Jul 28 09:40:11 [52] kCGErrorIllegalArgument: CGXSetWindowListTags: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:40:11 [52] kCGErrorIllegalArgument: Set a breakpoint at CGErrorBreakpoint() to catch errors as they are returned
Jul 28 09:40:11 [52] kCGErrorIllegalArgument: CGXOrderWindow: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:40:10 [52] GLCompositor: GL renderer id 0x0002260e, GL mask 0x00000003, accelerator 0x0000282f, unit 0, caps 0x00000003, vram 256 MB
Jul 28 09:40:10 [52] GLCompositor: GL renderer id 0x0002260e, GL mask 0x00000003, texture units 4, texture max 8192, viewport max {8192, 8192} extensions 0x0000000f
Jul 28 09:41:57 [52] kCGErrorIllegalArgument: CGXSetWindowListTags: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:41:57 [52] kCGErrorIllegalArgument: CGXOrderWindow: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:42:34 [53] Server is starting up
Jul 28 09:42:35 [53] CGXMappedDisplayStart: Unit 0: boot screen
Jul 28 09:42:36 [53] CGXMappedDisplayStart: Unit 0: no display alias property
Jul 28 09:42:36 [53] CGXMappedDisplayStart: Unit 1: no display alias property
Jul 28 09:42:36 [53] AGCAttach: Couldn't find any matches
Jul 28 09:42:36 [53] CGXPerformInitialDisplayConfiguration
Jul 28 09:42:36 [53] Display 0x4272800: MappedDisplay Unit 0; Vendor 0x610 Model 0x9ca0 S/N 0; online enabled built-in (0,0)[1280 x 800], base addr 0x1800000
Jul 28 09:42:36 [53] Display 0x3f003d: MappedDisplay Unit 1; Vendor 0xffffffff Model 0xffffffff S/N -1; offline enabled (2304,0)[1 x 1], base addr 0x1800000
Jul 28 09:42:37 [53] kCGErrorIllegalArgument: CGXSetWindowListTags: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:42:37 [53] kCGErrorIllegalArgument: Set a breakpoint at CGErrorBreakpoint() to catch errors as they are returned
Jul 28 09:42:37 [53] kCGErrorIllegalArgument: CGXOrderWindow: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:42:38 [53] GLCompositor: GL renderer id 0x0002260e, GL mask 0x00000003, accelerator 0x0000282f, unit 0, caps 0x00000003, vram 256 MB
Jul 28 09:42:38 [53] GLCompositor: GL renderer id 0x0002260e, GL mask 0x00000003, texture units 4, texture max 8192, viewport max {8192, 8192} extensions 0x0000000f
Jul 28 09:43:24 [53] kCGErrorIllegalArgument: CGXSetWindowListTags: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:43:24 [53] kCGErrorIllegalArgument: CGXOrderWindow: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:44:16 [53] Server is starting up
Jul 28 09:44:16 [53] CGXMappedDisplayStart: Unit 0: boot screen
Jul 28 09:44:16 [53] CGXMappedDisplayStart: Unit 0: no display alias property
Jul 28 09:44:17 [53] CGXMappedDisplayStart: Unit 1: no display alias property
Jul 28 09:44:17 [53] AGCAttach: Couldn't find any matches
Jul 28 09:44:17 [53] CGXPerformInitialDisplayConfiguration
Jul 28 09:44:17 [53] Display 0x4272800: MappedDisplay Unit 0; Vendor 0x610 Model 0x9ca0 S/N 0; online enabled built-in (0,0)[1280 x 800], base addr 0x1800000
Jul 28 09:44:17 [53] Display 0x3f003d: MappedDisplay Unit 1; Vendor 0xffffffff Model 0xffffffff S/N -1; offline enabled (2304,0)[1 x 1], base addr 0x1800000
Jul 28 09:44:18 [53] kCGErrorIllegalArgument: CGXSetWindowListTags: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:44:18 [53] kCGErrorIllegalArgument: Set a breakpoint at CGErrorBreakpoint() to catch errors as they are returned
Jul 28 09:44:18 [53] kCGErrorIllegalArgument: CGXOrderWindow: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:44:19 [53] GLCompositor: GL renderer id 0x0002260e, GL mask 0x00000003, accelerator 0x0000282f, unit 0, caps 0x00000003, vram 256 MB
Jul 28 09:44:19 [53] GLCompositor: GL renderer id 0x0002260e, GL mask 0x00000003, texture units 4, texture max 8192, viewport max {8192, 8192} extensions 0x0000000f
Jul 28 09:45:32 [53] kCGErrorIllegalArgument: CGXSetWindowListTags: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:45:32 [53] kCGErrorIllegalArgument: CGXOrderWindow: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:46:16 [53] Server is starting up
Jul 28 09:46:18 [53] CGXMappedDisplayStart: Unit 0: boot screen
Jul 28 09:46:18 [53] CGXMappedDisplayStart: Unit 0: no display alias property
Jul 28 09:46:18 [53] CGXMappedDisplayStart: Unit 1: no display alias property
Jul 28 09:46:18 [53] AGCAttach: Couldn't find any matches
Jul 28 09:46:18 [53] CGXPerformInitialDisplayConfiguration
Jul 28 09:46:18 [53] Display 0x4272800: MappedDisplay Unit 0; Vendor 0x610 Model 0x9ca0 S/N 0; online enabled built-in (0,0)[1280 x 800], base addr 0x1800000
Jul 28 09:46:18 [53] Display 0x3f003d: MappedDisplay Unit 1; Vendor 0xffffffff Model 0xffffffff S/N -1; offline enabled (2304,0)[1 x 1], base addr 0x1800000
Jul 28 09:46:20 [53] kCGErrorIllegalArgument: CGXSetWindowListTags: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:46:20 [53] kCGErrorIllegalArgument: Set a breakpoint at CGErrorBreakpoint() to catch errors as they are returned
Jul 28 09:46:20 [53] kCGErrorIllegalArgument: CGXOrderWindow: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:46:20 [53] GLCompositor: GL renderer id 0x0002260e, GL mask 0x00000003, accelerator 0x0000282f, unit 0, caps 0x00000003, vram 256 MB
Jul 28 09:46:20 [53] GLCompositor: GL renderer id 0x0002260e, GL mask 0x00000003, texture units 4, texture max 8192, viewport max {8192, 8192} extensions 0x0000000f
Jul 28 09:46:58 [53] kCGErrorIllegalArgument: CGXSetWindowListTags: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:46:58 [53] kCGErrorIllegalArgument: CGXOrderWindow: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:48:03 [53] Server is starting up
Jul 28 09:48:04 [53] CGXMappedDisplayStart: Unit 0: boot screen
Jul 28 09:48:04 [53] CGXMappedDisplayStart: Unit 0: no display alias property
Jul 28 09:48:04 [53] CGXMappedDisplayStart: Unit 1: no display alias property
Jul 28 09:48:04 [53] AGCAttach: Couldn't find any matches
Jul 28 09:48:04 [53] CGXPerformInitialDisplayConfiguration
Jul 28 09:48:04 [53] Display 0x4272800: MappedDisplay Unit 0; Vendor 0x610 Model 0x9ca0 S/N 0; online enabled built-in (0,0)[1280 x 800], base addr 0x1800000
Jul 28 09:48:04 [53] Display 0x3f003d: MappedDisplay Unit 1; Vendor 0xffffffff Model 0xffffffff S/N -1; offline enabled (2304,0)[1 x 1], base addr 0x1800000
Jul 28 09:48:05 [53] kCGErrorIllegalArgument: CGXSetWindowListTags: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:48:05 [53] kCGErrorIllegalArgument: Set a breakpoint at CGErrorBreakpoint() to catch errors as they are returned
Jul 28 09:48:05 [53] kCGErrorIllegalArgument: CGXOrderWindow: Operation on a window 0x2 not owned by caller SecurityAgent
Jul 28 09:48:06 [53] GLCompositor: GL renderer id 0x0002260e, GL mask 0x00000003, accelerator 0x0000282f, unit 0, caps 0x00000003, vram 256 MB
Jul 28 09:48:06 [53] GLCompositor: GL renderer id 0x0002260e, GL mask 0x00000003, texture units 4, texture max 8192, viewport max {8192, 8192} extensions 0x0000000f
I had a macbook 3.1 white with external VGA and i never had any issue during 15 months of use (with external screen 5 days per week).

External LCD Monitor Flickers for 10 - 15 minutes before it settles down

I have a Lenovo 3000 N100 with a samsung syncmaster 931B as the second monitor. When I activate the monitor, it flickers for around 20 minutes before it settles down. I have the resolution set at 1280x1024 32-bit color 60hertz refresh rate.
I've had this monitor for more than one year, without any problems. I've recently updated the nVidia Ge Force 7300 video drivers on my Windows XP Pro SP2 system.
Actually, the problem is not the monitor. I can connect this monitor to a second laptop and it works fine, with no flicker.
I have PC-Doctor 5 installed and the video tests for the nVidia GeForce 7300 and the external monitor itself ran with no problems.

I have a similar issue: external Samsung monitor worked fine on Vista, but I could not stand Vista. I put Win Xp Pro on... and I cannot get any signal out through the VGA port now. The monitor works fine (still), tested with two other latptops.
My suspicion is that the nVidia driver files, possibly the .INF, may need tweaking / replacement / etc.
Any assistance appreciated.

External VGA Monitor flickering

Hi, I own a t520 and from day one, my external connected VGA monitor Samsung has some flickering. Its hardly noticeable but in dark colors like black you can notice it.
Its only happening on the external monitor and I connected it to another laptop and the issue is not there. I only have this with my Thinkpad.
Is this a hardware problem?

Yes, you are right, I did not tested it with another cable yet, and its a cheap VGA one, but it does not do this on on a very cheap budget laptop. Maybe the VGA port on the Thinkpad is very sensitive. Its running on 60 Hertz. I only have 50 and 60 to choose. And I always run on the highest.

Screen flickering really bad after unplugging external monitor

Hello,
Since I updated my Macbook Pro 13' Retina (inch, Early 2013) to Mavericks, almost averytime I unplug my external monitor or TV, the screen starts flickering really bad. This is the screenshot of the issue. Sometimes you can see all the apps opens, but the "flicker" effect takes over the screen, making it impossible to user the computer. The only way I found to use the compuer again is restarting it. It's really weird.

Howdy inireugg,
We've an article that provides a host of helpful information and tips for resolving issues with displays.
No video or no signal, image distortion, "snow," or flickering
Check connections
When using an external display be sure to check the following:
If you're using an Apple notebook, confirm the AC power cable or adapter is securely connected to the computer and the cable providing power to the display is also secure. It is always good to have your notebook connected to AC power when an external display is in use.
Confirm display adapters are fully seated in their respective connections and that they are supported models and for the computer and display. Refer to these articles to assist you with adapter compatibility and further configuration information:
Monitor and Display Adapter Table
About Mini DisplayPort to HDMI adapters
Apple Mini DisplayPort adapters: Frequently asked questions (FAQ)
Thunderbolt ports and displays: Frequently asked questions (FAQ)
Remove all display cable extenders, KVM switches, or other like devices and retest to determine if the issue is resolved.
Try unplugging the video adapter or cable and then plug it back in.
If more than one video adapter is in use (or "daisy-chained"), troubleshoot by using only one adapter.
Example: A mini DisplayPort to DVI adapter connected to a DVI to HDMI adapter is an unsupported configuration because there is a series of adapters in use.
If available, try using a different display and or adapter (or use a different connector by using DVI instead of VGA, for instance).
Reset the systemYou can reset the Mac's parameter RAM and SMC.Reset the resolutionStart by resetting the Mac's parameter RAM. If the display does not come up, was previously set to an unsupported resolution, and still results in no video:
Start up in Safe Mode.
From the Apple () menu, choose System Preferences.
Choose Displays from the View menu to open the preferences pane.
Select any resolution and refresh rate that your display supports.
Restart your computer.
Apple computers: Troubleshooting issues with video on internal or external displays
http://support.apple.com/kb/HT1573
Cheers,
Allen

External monitor occasionally flickers

I haven't seen a post about a problem that seems exactly the same as mine.
My MBP is the 2010 version and is hooked up to an external monitor via DVI. The problem is that the screen briefly shakes, lasting no more than half a second. It does it sporadically with no regular pattern-sometimes its two hours between when it does it, other times its five minutes. It isn't a huge problem in terms of usability, but I paid too much for this computer to have a problem.
I know it isn't the monitor or the cables because I've tried different monitors and cables, including using different mini-DV cables.
Thanks!

How did you setup your display preferences? Under the arrangement tab, it will differ on which one is your main viewer. Not to mention where (which monitor)your tab menu is set. Do you have the right adapter for your external monitor?
When does it blink; when your starting up? Jon
I did go throught the display preference. The external monitor is located to the right of the imac on the arrangement tab.
I'm not exactly sure what you are referring to with how the tab menu is set. The display tab is set to 1280x1024, millions of color, 60 HZ (other option is 75HZ).
The color tab has a defaul profile of VE710s-2 which I just used from day one.
The monitor only has an analog in (no digital), so I'm using the apple adapter for the small square plug on the back of the imac to analog. I didn't think I had any other choices (right or wrong) for that item....
wwc

External monitor flickers

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