I had a similar problem with one of my systems recently. It would just reboot any time it felt like it, could be 1 minute or two hours after starting up.
I too was resigned to replacing the motherboard. I even considered that the OS was corrupt, however I have RH Linux on another disk and this would do the same thing.
As a part of the diag process replaced the PSU and lo and behold runs forever now. I guess that one of the power rails just started to fall off, probably the 5V. It's a fairly quick check, providing you've got a compatible PSU to test with.
Peter Mills