Ok ..good technically when the PDC goes down the BDC is supposed to kick in and run as PDC until the PDC server is back up , in a W2K enviroment there are no pdc or bdc just DC Domain controlers, therefore when the main DC fails anoth DC should still be able to authenticate users and profiles, this will give you the ability to repair the main DC and still have users log onto the network. one more question is the DNS and WINS and DHCP server all on the main DC machine.

In Short either way you look at the network you will need some downtime to further troubleshoot the server crashes. It would make life easier if there was another dc on the network so that users will not fail to authenticate...but not knowing totally how the servers are set up it would be safer to schedule downtime maintenance in an non-peak hour and apply the maintenance to the server that is faulting out. Got another Question, Have you viewed the event log on the server in question for any events that flag or pertain to the crashes on the server.