SBS 2003 throwing users off network

Hi

My SBS2003 started throwing people off the network. One minute they on, the
next they must restart to logon again. this is happening more regularly.
When they are working on a document and are disconnected they cannot save or
even click onto the domain name in the workgroup. They can ping the server
fine. I must either restart the wstat or server to reconnect.

Any Ideas ?

Thanks in advance