I have latest drivers for graphic card and mobo. I havent turned up anything in bios like CPU or RAM. A check on CPU temp after a reboot doesnt give any strange values (maybe temp goes down right after reboot?)
Recently had same problems with 2900+AMD with Chaintech MOBO. Restarts, temperature problems, and voltage warnings. Mostly while gaming.
I had updated Video card driver to the wrong version. MS OS 64 bit version.
Installed the correct Video driver and the problems resolved.