Whats going on with these 3COM NIC’S

I have three window 98 PC’S. All of which have experienced the same problem a number of times. The user will boot the PC and will not get the Novell login. I have tried to run ipconfig and get an error (I do not have the exact error) but placing another NIC in the PC fixes the problem. Today the same thing happened, I have saved all the previous NIC’s marking them bad because I did not really think that they were, and have not had time to test them. I gave one of these to my co-worker to install and it worked fine! What is going on with these NIC’s!!!!????