MapDrive to server hangs from time to time

In my office, there is one server running windows server 2003 R2 64bit service pack 2. The server is connected to 30 windows-based clients, I have created Map network Drive (drive z:) to a shared folder in the server. The client computers can access the shared folder, but only for some period (2-3 weeks or a month) then clients just can’t access the server, when click at the network drive z:, it just hang on there and make windows explorer not responding, then I have to restart the server, the problem solved. Anybody has an idea of what is the causes and how to solve this problem permanently.
Thanks