Hiya, and welcome to the site
At first guess I would of suggested your modem was at fault, but since that's been changed for a new one, and you've tried USB.. then I think that can be outruled.
Maybe the signal levels are too high/low? Worth giving CS a call and mentioning this? Give it a few hours, see how it goes (I'd get back onto your NIC though) and if it continues, then give CS another call..
You havn't made any noticable changes to your PC before this problem?