We're having a problem with users losing connectivity to our NetWare 6.0 SP5 server. They lose mapped drives - including access to their home directory, which results in Word shutting down and loss of data. It happens across all subnets (6 virtual LANs), Windows XP and some Windows 98. It doesn’t matter which NIC they have installed, so it doesn’t look like hardware.
- David Wojtowicz

Based on what you're describing, it could be either a hardware or software problem. I would first look at the thing(s) that are in common with the problem. Start at the server and work out from there. Check that you have the latest Novell updates applied to the server. See if you have the latest network card driver installed on the server. Look at the server logs to see if there are any entries during the times people lose their connection. Talk to users and see if there are particular time(s) the connection is dropped. It could be something as simple as power management to the network card on the workstations not being turned off. This causes the default Windows power management settings to turn off power to the network card because the mother board temporarily shuts off power due to inactivity.

Go into your network switches and look for any interface counter problems that would indicating networkwide problems. Start with the port the server is connected to and work your way out from there. Even with updated network card drivers for the server, the problem could still be the network card in the server. You mentioned that you tried an alternate network card in the workstations but didn't mention if you tried the server. This is definitely worth trying and isn’t too expensive. If you have the luxury of doing the following, it is worth a try - connect a workstation directly to the server using a crossover cable. If the problem goes away with a directly-connected workstation, this would seem to indicate a problem with one of the switches on the network. If you don't have some type of surge protection, or preferably a UPS on each of the switches, now would be a very good time to install it. It helps prevent a power problem from being the cause of what you're seeing on the network. Try a different network cable from the server to the port on the hub/switch it's on.