Users going offline.

Ok here is the situation I have 3-5 users who's workstations go offline to
our Windows 2000 file server, which is running service pack 4. It is not a
consistent issue it occurs every couple of weeks and then the issue will go
away. It has come to the point the only thing I can do is rebuild the
server. Here is more info.

- When the user goes offline it is only to the file server the users are
able to reach the other 20 servers that we have.
- I can ping the server and I can resolve the name of the server via
nslookup.
- It does not happen to everyone only about 3-5 users, we have about 35
users on the network.
- I have tried disabling offline files locally on the computer.
- I have tried disabling offline files through group policies.
- I have had the a user try a different computer and it stills occurs
- I have had one user login to two different machines at the same time it
occurred on one but not the other.
- I have tried hard coding the ip through the host file.
- The 3 - 5 users all seem to go offline at the same time.
- There is a login script that runs for domain users that maps 3-5 drives to
this particular server for everyone in the company. I have tried changing it
to map the drives via ip instead of name.
- When I synchronize the files the users can continue to work normally, but
the workstation can go offline every 6 mins to every hour.
- The issue seems to resolve itself over a period of 3 days.
- I can also re-establish a connection if I map a drive via the ip vs. the
name of the server.

Anyone have any suggestions?