Communications lost

We are running Win NT 4.0 SP6a using 1 3COM NIC card. 15 workstations they also either use 3COM or Linksys NIC cards. For some reason, 50% of the workstations have lost communications with the server out of the blue. On investigation of the problem,found that half of the workstations no longer showed the drives on the server mapped. The server would allow some of these to reconnect upon remapping some not. What could cause this to begin with and what could possibly be the solution? I have spoke to several tech support people but they are puzzled as much as I am at the Gremlins…..