Windows 2000 Shutdown Weirdness

Hi, I have a problem with a Server Running Windows 2000 Ad Srv [SP 3] Essentially the server will be running fine for around 3 hours – and then it will shutdown for no reason (full shutdown and power down).
Nobody is doing it remotley, there is no power problems in the room – nor any hardware problems.
I have eliminated DIMM problems – so it does seem to be something to do with the server software.
The Server is a Compaq ML 570 2.6 GB RAM x 4 Zeon processors running E2K SP 3.