The overclocking issue revisited

Today, about 5 days after first having the problem with my processor overclock not working anymore I found the time to do some testing.

I shall repeat the issue:
CPU was overclocked for nearly 3 years without an issue. About a week ago, about the same time I received my Display Port capable HP Monitor my PC would only boot when not overclocked. Otherwise it reset itself right after the memory check / message showing the "Memory runs in Dual Channel Interleaved". This is right before the detection of SATA/IDE devices. It did't matter how minor or major the overclock was. Even it technically wasn'T a overclock it still wouldn't boot completely.

My configuration is:
Intel Core2Duo Conroe E6400 2.13ghz (overclocked to 3.2ghz)
Gigabyte 965P-DQ6
2*1024 Corsair XMS2 8500
2*2048 AData Vitesta
Asus Radeon 5850
2* Seagate Barracuda 7200.10 500gb
Samsung F2 640gb
Sasmung F3 1000gb
Cougar 750W Poer Supply
Aerocool ExtremEngine 3T (which is annoyingly small)

Well, first I tried to check the Ram, while Memtest86 didn't find any issue with it I decided to better be safe than sorry.
Taking out both 2048 sticks it booted... hm!
Installing one of the 2048 again - no boot.
Taking out both 1024 sticks while leaving both 2048 in - no boot.
Installing just one 2048 - booting.
Hm... so I guess it ain't the ram after all.

Well I remember not yet feeling quite at ease with the F3 which found its way into my cramped case just before Christmas. So lets take 'em out. Disconnecting all HDDs but leaving all the ram in - no boot.
*sigh*

Well... what else is new? And what did I change right about the time the trouble started? Yeah... that's right, the Display Port-Screen arrived and I connected it instead of the DP-VGA Adapter that lead to my TV.
Hm... Well, so let's imagine that this might be the general area of where to look.
So what's the difference? The TV was never turned on when I turned the PC on. The Display Port capable HP Monitor on the other hand always was.

Lets have a look... or rather no look and disconnect the Display Port. Surprise, surprise, it lives! Reconnecting the DP-Cable and leaving the monitor off, it boots again. Turning the monitor on, no boot.

So there we go, for whatever reason there might be, overclocking a Core2Duo in a Gigabyte 965p-DQ6 mainboard with more than 2gb of RAM and a Radeon 5850 and a Display Port connected device that is turned on results in not being able to overclock anymore. Brilliant!

Anyone care to explain this phenomenon to me or if it happens in different configurations as well?