Reboot Revisited

I have a PII-233MHz computer that is rebooting just after connecting to the internet or after about 30 – 45 min of use. Doesn’t matter what is going on, it just reboots. I’ve replaced all cards in the machine and the drive and ram. The only thing left to change out is the motherboard and CPU. Any ideas why this is happening??

I should note that all previous drivers were uninstalled when the cards were replaced.