large amounts of data >connections lo

I am running a MS SBS2000 server RAID5/fiber switch -> connected to MS clients (mix 98/2000) using 100Mbit LAN. When copying large amounts of DATA across the network from one client to the server (or <>) all clients suddenly lose connection with theserver. The server itself does not hang everything looks ok. I have to restart the server before users can do a logon .. anybody had this ?