IMac always auto detects for external monitors after waking from sleep or from display off

Question

I've got a Dell 24" monitor (2408WFP) plugged into my mid 2011 iMac via DVI cord and thunderbolt attachment. Things work fine.
When I need to leave my computer, I usually turn the displays off using the shortcut keys: CTRL + SHIFT + EJECT
This works perfectly and both displays go black. However, when I return to my computer and reactivate the iMac, the iMac display will turn blue as if I have just plugged in a new monitor even the external monitor is still plugged in. It usually flashes blue twice before displaying the password prompt. I can usually just log back in as normal. I've never had this happen before using Snow Leopard. It's annoying but strange.
However, even more strange is that sometimes when returning to the desktop, the iMac will do the auto detect display thing and then when I log back in, the external display just shows static as if there is no signal. I've checked the cable a million times and everything is as tight as tight can be and both ends are plugged in properly. Unplugging the displayport adapter from the iMac and then reattaching it restores the desktop on the secondary display although sometimes, it results in the secondary display showing a gray background picture instead of my normal picture. Replugging the secondary display seems to fix that too although sometimes it doesn't.
Why is this happening?
- Problem with the adapter?
- Problem with the DVI cable I'm using?
- Compatibility issues with my external monitor?
I need to stress the point that this has never happened before with Snow Leopard.
If I am the only one experiencing this then I suppose it may be an issue with the iMac or adapter or even the cable. But if anyone else can shed light on this problem, I'd really appreciate it.

Answer 1

Hi Micheal,
I've been experiencing the exact same issue with my new iMac I just bought.  I bought it refurb so I wasn't sure if it was a defect with the hardware or not.  Since I'm not the only one I will asume it's not for now.
Setup:
- iMac 27-inch 3.4GHz (Release May 2011)
- Dell 2709W external monitor
- Hooked in with mini display port to DVI adapter
- Lion 10.7.3 all up to date to this day
From what I've been able to gather the issue presents itself only when my Dell 2709W enters "Power Save Mode" after I do CTRL + SHIFT + EJECT or when the diplays simply goes to sleep by themselves.
Although if I hit that combination and the displays goes to sleep but I don't wait long enough for my Dell monitor to actually "Power Save Mode" (wait like 2 sec) then touch the keyboard then the 2 displays (iMac and Dell) resumes from sleep without the issue presenting itself.
I'm tempted to buy the mini-display port to HDMI just to see if the issue is still present with that adapter.  If I do I'll update my post with the outcome.
If anyone else has any info to help with this issue it would be greatly appreciated.  It's very annoying when you resume from sleep and you find all of your windows that where opened on the external display before sleep squeezed into your main display (iMac).
Thanks!