My suggestion? See about going to a older version of your Mobo. Sounds to me like the update to your motherboard (assumeing your network card is integrated) was the culprit.
My reasons are:
You said the internet works fine till you plug up.
You said ti worked fine before - (so the card is ok)
When you plug into the modem you get issues as well (so its not the router)
DSL Modems are really hard to mess up.. there pretty simple devices, and besides you said it works so long as you dont connect your pc
My best guess goto the older version of your motherboard and try again. i think it would be fine.
Also try to update the firmware on your router, Call AT&T and make sure the DNS numbers on your modem are valid.