Workstation “loses” contact with server

I’m connecting XP pro (sp1) workstations to a windows 2000 domain containing three servers. Intermittently, some pcs seem to lose connection to server1. I get the message “You are working offline on “server1” and may continue working normally. Changes to offline files will be copied back to “server1″ when you synchronize”.
All pcs are permenantly attached and I don’t get a similar message from server2 or 3. If I ping the server while this message is showing I get a response. Any ideas? Is thisa pc or server problem?