CPU failing?

Hello all, first time poster.

I'm having a weird issue with my computer that I have never come across before.
My system works fine when I am just surfing the web, using Microsoft applications, etc. However, whenever I try to play any game my computer has started freezing randomly. It doesn't freeze completely but it will freeze for about 15 seconds, then work for a second, then freeze again, etc. etc. I check my CPU performance when this happens and of course it is at 100% but my memory usage is not that much.

I've blown out my HDD and reloaded Windows semi hoping it was virus related but it still happens. I've run a burn-in testing program and at full load(CPU, RAM, Graphics) none of my components fail. I had thought maybe my heatsink was failing and the CPU would overheat but I left it on all night, pulled it apart today and the CPU was not even remotely hot. It's only games that really cause this freezing.

The pertinent components in my system are:
AMD 64 X2 Dual core 4600+ CPU
2gb RAM - 4 512K sticks
GeForce 7600GT video card
Windows XP Pro

I should also point out that I built this system about 2 or 3 years ago and never had a problem until just now.