Just got my new iMac last weekend, came with Mountain Lion, etc, pretty cool..except from the get go I had the ethernet connection problem. Well I have searched around, read all the tips, and still nothing. I knew that the ethernet port on my Mac was good, becasue when connected straight from the modem it worked, and, also knew the router was good, because connected to other things it worked as well. Not to mention every now and then it would work on the iMac.
Now, it has been working fine since my brother (IT major, etc.) remotely accessed my mac and was over the phone with me explaining everything as he went, he tried multiple things, and still got the same, "cable unplugged" error, connected then no connection..etc. He tried EVERYTHING, he really wanted to give up, but luckily for me, he didnt.
He wanted to try one more thing. So, again we disconnected the ethernet cable from the Mac, re-created an ethernet connection, left all the settings like they were originally, automatic etc., click on the ADVANCED... tab, select the 802.1x tab from the top menu, and on the lower left corner it says "Enable automatic connection", this box was checked. UN-check it, go back to settings hit apply. We reconnected the ethernet cable and it has dropped since.
I dont want to jinx myself (knock on wood) lol, but I think its worth a shot. He did explain what the whole 802.1x means and what it's for, but honestly I didnt care. So this may be an option to you guys out there. If it has been posted before, I apologize, i did search around and did not find anything on this, and this is my first post. Hopefully help some people out..