RPC Service Unavailable

I have a Windows NT Server 4.0 which is the corporate Intranet Web Server. Every 2 or 3 weeks, I get a call that clients are not able to connect to the server. When I go to the server and try to login, I get the Logon Dialog Box without the tab for DOMAIN in it. I am not able to control the services remotely. One day this happened while I was in the server thru PC Anywhere and I saw that RPC Service has failed and at that time I was not even able to re-start that service. What could be the reason for RPC Service to fail and stop responding?