Sometimes, random reboots could be a result of cpu temperature. I've had machines that overheated do that exact same thing. Adding more fans or a different cpu cooler usually solved the problem, when it starts doing the reboot thing, check the cpu and see if it's real hot. If not, then it could be a memory thing as well, though I'd expect that to not wait for hours before manifesting itself, but I've seen memory cause reboots too from time to time.
