I've just built a new PC, with the following spec,
ASUS M2N-X Plus GeForce 6100 Socket AM2 6 channel audio ATX Motherboard
AMD Athlon 64 X2 6000+ Socket AM2
4GB DDR2 667MHz/PC2-5400
XFX 8600GT 256MB DDR3 XXX edition Dual DVI TV Out PCI-E Graphics Card
Seagate ST3250410AS 250GB Hard Drive SATAII 7200rpm 16MB Cache
Casecom Silver 500W ATX Power Supply - 20+4pin ATX12V 4x Molex
But when i play any game COD4, UT3, Crysis, etc. It starts off with good frame rate, on UT3 i get 62 FPS, but every 40-50 seconds the frame rates drops to about 2-3 for a few seconds then goes back up to 40-50, if i put all the graphic settings on low it still happens. I think i have a hardware problem but not sure??
If i set the affinity for all the windows apps to CPU 0 and the game to CPU 1 it still does it but not as often.
First check to see if you have a program running that's taking up CPU time. With the computer idling, go to the task manager and see if you have any spikes in your CPU usage, also check how much memory your system is using, you might have something hogging resources. You should also check for updated drivers.
My guess is that its a software problem. If you find something in task manager, try to locate the offending program and stop its process and see if that fixes the problem.
Check again. This looks like a software problem indeed. Something is running in the background, like your antivirus, disk defragmenter, windows update, windows restore feature, I don't know, but something is. Disable your internet connection, check again and see what is running when the slow-down occurs.