Momentary loss of shared drives

We are running a total Windows 2000 network. Periodically users will lose connection to ALL shared drives. It happens randomly and I have been unable to track it down to its root cause. Most of the time, the users’ shared drive connectivity will almost immediately return when they attempt to access them. When it happens, an error message comes up on their screen complaining about losing connection to the shared resources. (I will try and capture one, but they are random in nature)