crashing

Okay here is what is happening: My computer works just fine until I start a game, then it runs for about 2 minutes and it restarts itself. I can search the web and do other things, just crashes on games.

This is what I have tried: I have made sure that I am keeping the components cool (99f) core, case (101f). I tries upgrading my memory, pulled my sound card, (used onboard), pulled my video card (Used onboard) and still it crashes. Someone sugested a more powerful power source (I have 450 watts) and I am not sure if re greasing my thermal grease on the chip would help although I had a problem with installing the fan unit and it was a little messed up, but would this be the cause as I have made sure the chip is running cool by checking in bios and with everest(or everwood whatever that program is called. I am very frustrated, but would rater ask if someone may have had a simular problem and a direct cure! Is maybe flashing my Bios a cure? I get a message that it is hardware, but I have pulled everything, so maybe I damaged my motherboard? Help me Obi-one, your my only hope!

P4 d processor 3.2 mhz
ecs motherboard(cant remember here at work which one) has Agp slot x4,x8
running 1gb ddr2 533mhz memory (PNY)
gforce 7800gs card
soundblaster x-fi extreme music

I have read various post, and tried and it seems I'm not the only one here, and it litrally crashes only when I play games!!!!

try a memtest.