I recently had that problem (reboot upon heavy load) and it turned out to be a bad processor. Random reboots, though, can also be caused by overheating, bad RAM, bad power and bad mobo. Google "memtest" to find a program you can use to check your RAM. Assuming RAM is good and you have ruled out overheating, try to swap out your power supply and see if that helps. If not, chances are it's your CPU.