This could be because the Ethernet card or cable connection is not working
correctly. Here are four trouble-shooting steps:
1) Make sure that the Ethernet cable from the Ethernet card to the rest of the
network is connected and seated properly and snugly.
2) Check that there is not a broken connection in the main network cable
itself.
3) Check that cables do not show any physical signs of damage. Ethernet cables
should not be crimped, bent, or squished.
4) Replaced all cables that show excessive wear.