Unreachable Workstations

I co-administrate a small (8 workstation/peer-peer/no domain)LAN and lately we have been have an irritating, if not interesting, problem.

All our machines tie together via a hub and a hardware firewall and share an ISDN connection to the internet. I have checked and double-checked all our TCP/IP settings and everyone is able to access the internet. HOWEVER sometimes, and this seems to be on a random basis, some of our workstations become unreachable via My Network Places ie, even though the PCs can be SEEN, when double-clicking on them I get messages like, “network path not found” or “server is not configured for transactions” and one computer is just not browsable AT ALL.

I know that there is probably something obvious that I am overlooking but, I just can’t seem to find it. Any suggestions would be GREATLY appreciated.