Unusual Problem in MW2 and World at War

I recently...improved...my computer. Sort of. IF the problem is the mobo, I will return to my previous Intel D915GUX

I have converted to using a motherboard from a discarded HP DC7700 CMT system. Since the board is 100% OEM, I have no way of knowing the actual board name...at least one that I know of.

I still use my GeForce 8500GT max overclocked etc
3GB 533 Mhz RAM (in Dual-Channel)
but I also upgraded the OS to 7 64-bit to accommodate the Pentium Dual Core on the mobo.

Everything went smooth, but this is the problem:

Firing up MW2 with the following settings:
1080x728
No Shadows
Specular Lighting
4x AA
AF at tops trilinear filtering
Full textures etc

Every 20 seconds or so it freezes. The only way to get out of the freeze is to hit CTRL+ALT+DEL so the Windows menu appears, then Cancel to go back to the game.
And it happens over and over again. Same thing goes with World at War.

I even try things at lower settings. World at War did something freaky: starting a new game and when it got to the part where the Japanese officer puts the cigarette out in the Marine's eye it completely froze and took over my screen. I hit CTRL+ALT+DEL, it went to the Windows menu, then right back to the game frozen. Had to do a cold boot to recover.

To make it even weirder than it already is. Sometimes in the pause menu, the game freezes, then flashes to the game still frozen mind you.

Also MW2 has weird purple, blue, etc color errors when it freezes.

Any idea what could be causing this?

Should I go back to my Pentium 4 and Intel D915GUX?
It worked perfectly on there...I was getting upwards of 35FPS average.