Getting Thrown off the Network

I have a Windows 2000 Server Box. I have 12 Windows 98 and Windows me workstations attached to it via shared Routers, Hubs, Switches. The Problem is, At Random, users keep getting bounced off the network. From a workstation that has been thrown off , I can ping the DNS,Server,IP’s, Gateway and surf the internet. The only thing I can’t do is Browse my network drives (Mapped Drives).
I would then have to reboot the computer and re login. Then all is fine until some one else or the same workstation gets bounced off again.
What do u think could be the problem.