can't connect via ethernet

hey folks. I have this late 2009 iMac, and for some reason i can't connect to the internet over ethernet.
in system preferences it says the cable is unplugged (it's not).
in the "about this mac" system report it shows the ethernet card is detected.
once in a while it suddenly says I'm connected via ethernet, but in reality, i am not. it just acts as though the modem is not connected.
I have done a pram restart, with no change.
the router is working fine. i can connect wirelessly, and the cable itself is fine. if i plug any number of other devices into it, like MacBooks etc. it works fine, so its not the router, and its not the cable.
Ive also rebooted the router, done a full restore from time machine, I've restored all the routers settings.. still no go.
any help?

 

its a basic ethernet cable.. is that what you mean?