Monitor turn off when playing call of duty 4

Hi,

I've got a major problem with my gaming...

My monitor ONLY turns off when playing call of duty 4!!

Hardware: (Bran new) 9600 GT graphics card, P5K-E motherboard. 4Gb RAM, 500W power supply, E6600 core 2 duo processor and WD 750Gb 32Mb catch sata hard drive.

Steps made so far...
I've placed a heat sink on the northbridge chip to prevent over heating, matched the RAM, replaced graphics card and power supply, I also added extra cooling fans & downloaded lastest patches for the game.

I'm sure it's not over heating, it's not the graphics card, no problems show on hardware diognostics.

Can someone please advise me on what could be the problem and the best solution to solve it?

Any feedback would be appriciated

Thanks