The problem is not USB, or anything like that, the problem is the firmware in the Ambit modem dies under certain circumstances, usually heavy load thats generated by P2P programs. I have only managed to alleviate it by placing a limit on this load via my FreeBSD gateway. I get the impression mrlipring likes his BitTorrent or eMule
I can bet my bottom dollar that if you hooked up an SB4100 to his line, it would work fine. You can get them on ebay (non hacked! perfectly legal) for about 40 quid, you just have to convince someone at NTL to register it's CPE MAC instead of the Ambit's. The Surfboard also has a better status log than the Ambit's which would be very useful for diagnosing any kind of line fault.
Just out of interest, what is the software and hardware version on your Ambit ? Mine says:
Software version : 2.67.1011 Hardware version : 1.15
You can get this from the modem status page at
http://192.168.100.1 - if you can't get this page, reboot the modem (yes, after a while, the modems internal http server fails too!)