Have you checked inthe bios of the motherboard most of the amd boards have a option to reboot if the temperature goes above a certain level.
My computer will turn itself off once or twice a week, or will reboot for no apparent reason. I thought it my be getting too hot, so I put a fan on it. The CPU temp stays near 100 now, whereas it used to get up to 125. I have an Athlon XP CPU, and I'm running Windows XP Pro.