Loss of Drive mapping

We have a couple of PC’s that are losing there connection to 2 CD towers. When they log into the system it will show the drives under Windows Explorer, a few minutes later you can go back into Explorer and see that the drives are displayed but theyhave RED X’s through them. However, if you expand the drive, it reconnects to the system.

This process goes on all day long.

It is a Novell 3.12 server and Gateway boxes running WIN98.