USER UNEXPECTEDLY LOGS OFF

I have this one user that for some reason keeps losing access to all of their network resources. We have swapped 3 machines, ports on the hubs even cubicles. They will try to save a file from excel into their home drive but they are not allowed access and then find out that they have been disconnected from the network. I am running a complete NT 4.0 (servers), 95(clients) network. Any help would be great.