Well this can be 1 of 2 things or both.

1) bad NIC
2) bad enet cable

I would suggest checking the enet cable and verifying its secure in both your computer and router, after that if its still doing it. Replace it.

If the problem still occurs its probably your NIC.

There is also a chance its your router/switch that its connected to, in any case you can just try another port in the device.

If you dont have a router/switch and your computer is direct to your modem then.... SHAME ON YOU!!!

hope that helps...