Well, most likely it is hardware. I've known a semi-faulty PSU that did it, also could be the hardrive, cable, controller.
Well, it's not so common anymore (its a throwback to the old DOS days) but you could be slipping past the os safegards and overwriting a protected area of memory sparking a reboot.
You live in California? It could be a rolling blackout...
Just
how often does this happen? Does it ever happen while inactive or only when it is activly being used? Does it spark Scandisk? If it doesn't (and you don't have Tweak UI stopping it) look for a program to be doing it.
Once my live update went nuts and ever time my scheduled live update even happened it downloaded a tiny patch and prompted for a reboot (it never realized if it succeded so it kept trying over and over).
Got any spare parts lying arround to test? Try to narrow it down (Hate for you to buy an replacement part needlessly).
Oh..one last thought, you consider the power getting to the computer? Do you have a UPS (backup battery)?