Windows 2000 network problems

My client has a Windows 2000 PC. This is a computer in which I placed a new hard drive and installed Windows 2000 SP2. It is on an Ethernet network with a 3com 3c905b-tx NIC. It seems for no apparent reason, but especially when I am downloading a large file from the Internet or accessing a large file from a network share that the computer drops its connection. There is no ‘Network Cable Unplugged’ message. The only remedy is to restart the computer. I had an electrician test the wall jack to the patch panel. I replaced the NIC and both cables, to the wall and to the hub. I also downloaded the 3com nic diagnostics which say that the nic and network connection are fine. I do not want to reinstall the OS. Help!