I have seen an issue like that when using an Ethernet cable whose twisted pairs were not the correct ones (wrong strands were twisted together) in my ISP support days. This caused a problem with one NIC, but worked fine with another when connecting to a cable modem . Such a cable, almost certainly a handmade one, will pass a simple continuity test, so it cannot be easily detected unless using more sophisticated equipment .

darry wrote on 2020-06-15, 01:50:

I have seen an issue like that when using an Ethernet cable whose twisted pairs were not the correct ones (wrong strands were twisted together) in my ISP support days. This caused a problem with one NIC, but worked fine with another when connecting to a cable modem . Such a cable, almost certainly a handmade one, will pass a simple continuity test, so it cannot be easily detected unless using more sophisticated equipment .

I've tried different cables. Funny thing is, it has another RTL8139 NIC on a PCI card which works fine.

There is probably something wrong with the onboard NIC, then .

Considering that the mainboard can host a relatively fast P4, I would considering using a Gigabit NIC in it anyway . Even in a PCI bus constrained scenario, you will probably get 2 to 4 times more effective throughput out of it anyway (depending on Windows version used) .