Yes I had a similiar problem with my ntl cable modem and BT router. I rang their helpline too and was on a good hour and following their advice got it working in a fashion but if I had both laptop and desktop working at the same time one or the other would drop the connection.
Eventually frustrated at this I took the damn router back to PC world who advised me (and this is my advice to you, if its applicable)that on some ntl modems they will have both usb & ethernet sockets, in any event the usb cable should be totally disconnected as the flow of internet traffic will always preferably select this route over any other?? So they say. Still I tried it when I got back and hey presto it worked. BUT I AM NO EXPERT SO READ THE ABOVE WITH CAUTION.