Windows 2000 network connection

I have this problem on two machines;

I upgraded one system from nt4 to 2000 pro.I connect to the network but after a few minutes I loose the connection and get a hardware error. In order to re=activate the link I must reboor. I changed network card, still the sme problem. I uninstalled the protocols and re-installed them, still same problem.

Does anybody have any suggestions on how to solve this problem.I need those boxes for the programmers.