does it constantly do that? If so:
1) might be a bad termination on the jack (either on the PC side, or the router side)
2) could be a bad cable run (from the router to the PC)
3) could be a bad NIC
I would start with reseating the NIC, then try re-terminating the cable (if you can). Then try a new NIC. If that doesnt work, then replace the cable that is run from the router to the PC
One of those things SHOULD fix it....good luck