the only thing i can suggest is to ...
... check the hardware for faulty parts
... check for viruses / malware
... check for power line faults, install line filter and / or "battery power" (oops, right word?)
... install all patches and service packs that are available via windows update
... ask in news:microsoft.public.windows.server.general what and where to search in system logs for getting some hints
greetings from germany
Your check list matches mine. We are keeping it up to date and have temporarily removed a suspect UPS/battery backup unit to see if that fixes the problem. It's nearly brand new, so if we have to replace a power supply, it's still under warranty fortunately.
MemTest86 your RAMs.
I've had the same thing happened on my personal PC about 2 months ago. Dang thing would just reboot after a few hours. Before that it was running fine wher eit was on 24/7 for about 2-3 months straight between reboots.
It was a bad stick of RAM...
A call to the MFG tech support pointed me at a tech article documenting the power requirements of this machine. It clearly shows the current UPS that we have is inadequate.