Windows Monitor randomly going into standby mode, then PC rebooting

I'm using XP Pro, on a Tyan Tiger MPX with only one processor installed at the moment, and 512Mb of registered DDR.

Since installing XP (or my new processor (gone from 1.5 to 2.5Ghz), they were at the same time) every now and then the screen goes black, monitor goes into standby and then it reboots itself. Say I have winamp running when the screen goes black, it carries on playing, which I thought was odd. I've reinstalled SP2 and the latest NVIDIA Graphics card drivers since I thought it might be that.

Has anybody got any ideas?