Windows 2000 Server crash

I’m running Windows 2000 Server opt pk 2 on AMD 1.2ghz, 256 pc100 ram, 2 Western Digital HD’s in mirror image as a member server on a small network. Every night the server “freezes”. There is no apparent outside power problem, I have a 7 min ups set up. I have disabled all power scheme’s in bios and os. During the day when network is active, there are no problems. Event viewer does not help.
Any suggestions would be appreciated. Tom