I was told oncethat it happened because it was a type ATX computer. I suppose your is an ATX too.
I have a 2.2r2 (2.2.17) Potato on AMD 800 256 and Nvidea Geforce MX 32 MB and from time to time it starts spontaneously; usually after halting the system and pushing the on/off button off. After a few seconds it reboot by its own, but not always. It depends on the care i put on the finger when i switch off :-) I have to push the on/off button a little longer than i usually did with my old P200 and it switches off properly. But from time to time it seems as if a little wire were over heated and had enough energy to re-start. Most of the time works properly. I don`t care because i put a another energy switch. hope it helps Regards