shutdown by it seft

Is the server setup to automatically reboot on system failure? (system properties/advanced/startup and recovery)
make sure you have 'write and event to the system log' checked and that Complete memory dump id selected in 'Write Debugging Information'. This will give you some data to work on when the system reboots.

Can you tell from the Event log if the server is being shut down or if it is crashing? Check the messages just prior to the ones for it starting up and see if it shows the eventlog service being unloaded.
I have had problems before with admins connecting to servers through terminal server or PCAnywhere and accidently rebooting or shutting the system when they meant to log off.

start a log. write down time and date of reboots. anything else you can see to go along with it. does system get slow before rebooting? check basics, look at performance monitor. is swap file maxing out all the time? check your RAM.
this sounds like resource leak from faulty program or driver...turn on performance monitor to check for resources. do they drip out and finally the thing reboots? what has changed? did you update any drivers? did you fiddle anywhere with anti-virus software? do youhave a firewall running? what runs on server? exchange? iis? sql server? do those things have latest service packs...does win2k have latest service packs? does it have windows updates occurring automatically? is drive defragged? is the server on a ups? is the ups checked out?