The error you are discribing isn't wouldn't have anything to do with your computer at all. You should be able to take your cable modem, not even have a computer hooked up to it, turn it on and it should sync up with the server. If it isn't syncing correctly it's more than likely something with the line coming in. Cable internet service has a very low tolerance between the frequencies that it can come in through and if something interrupts that it can cause some problems. From what you've done I'd say try calling AT&T and they should be able to take care of it for you.
-Juxel