Ethernet cable to downstairs

As the downstairs PC is working from one of the ports I would try the following -
1. Plug the new cable from the PC upstairs into the port the downstairs PC is currently in
2. Run the new cable from the downstairs PC to the router

This will diagnose if the port you are using is faulty or the new cable.

Another thing, have you ever used the upstairs computer with an ethernet connection? This could be a driver/hardware issue of the ethernet port on the PC is not.