Losing Network Connections….

I have an office with 10 95/98 clients, Unix server, and 3com hub. I am running TCP/IP protocol, overall it is a peer-to-peer network with open telnet sessions to the Unix server. Sporadically all computers lose connections to each other. The hubhas been replaced already. I have a variety of nic cards out there: 3com, netgear and intel. The wiring is new. I think I may have a bad nic card but how do I know which one without replacing all. Any ideas??