I suggest you go hunt out a cheap PCI network card, and see if that fixes the issue. Nvidia's chipsets have never been the best, and it may well be an fault in the onboard adapter. Even a basic 10/100 card will do the job to troubleshoot the issue.
Scrap that, reading the OP for the win!
You have 3 possible problems there, cables, router and motherboard. Nvidia chipsets have never been the best, and it's possible that's going.
Reset the router to factory default settings, and then try it.
Have you used the same ethernet cable on each NIC? If so, find another one to try, or, even better, move the base unit to where there's a known working machine plugged in (if any) and try it on that cable.