Broken connection Win 2000 Pro

When trying to download big files from File Server to some workstations they lose their connection with network. All the workstations in my network are the same configuration. But only few have this problem. The only way thet I can conect them to network again is to restart them. All the computers in the network are Win 2000. SP-1 is installed on all of them. I would apriciate any sugestions.