Have similar problems on TWO Thinkpad T410 and a Thinkpad W510. In one T410, the nic reports the network cable is not connected (it ism and is good/connected), in the ohter the connection is unreliable, will drop every once in a while when connected to its laptop base (which works with other laptops, also tested on other good bases). On the W510 it just does not work. In all cases, drivers were uninstalled/ reinstalled/ updated, and even previous images of the system when the nic was working were restored.
So it seems there is some kind of hardware rot taking place there. The wifi is fine, and a workaround is to use an USB nic connection, somewhat annoying.
I wish there were some software tool to diagnose /confirm this is a hardware/ firmware issue.