ok, then let's check the settings Network within System Preferences
under Ethernet; click on Further Options and select the last tab to the right, which should read Hardware
first line lists the MAC address of your ethernet card
2nd line labelled Configuration, should read Automatically
3rd line labelled Speed, should read 1000baseT
4th line labelled Duplex, should read Full Duplex
5th line labelled MTU, should read Standard (1500)
If there is any problem, try deleting the Ethernet entry in the list of network devices on the main network settings page, restart OSX, it should report "new hardware found" and allow it to configure. Maybe that can clear the problem.