Connection to server

Good day,

We are running windows 2000 server and 2000 advanced
server at different sites the problem is occurring at both
sites.

The users machines on the network are Win98se machines.

The problem I am having occurs on different win98se
machines. After logon you could be working for hours and
suddenly the connection to the server is gone. You can't
open mapped network drives or see the server from Network
neighbourhood. The only way to fix it is to shutdown and
restart the computer. What could be causing this and how
do I fix it?

Thank you,