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).

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