Losing connection to File Server

I have several workstations that occasionally lose their connection to the Windows 2000 file server after they have been idle for awhile. Files on the server are no longer accessible. You have to reboot the workstation and you can log on again. (A simple log off/log on doesn’t work.)The operating systems of the workstations range from Win 98 to Win ME, and have different brands of nics. Two different 2000 servers do this. Microsoft admits this is a problem with 2000, but won’t give a solution online. All machines use NetBEUI protocol