There can sometimes be problems when switching from USB to ethernet.
The modem must be power cycled when changing from using USB to ethernet (or vice versa).
Also, some people, who use USB, actually find it necessary to leave the cable modem turned off overnight before then trying to use the ethernet connection instead.
Robin Walker's CM tips site has some info on USB>Ethernet swapping:
http://homepage.ntlworld.com/robin.d...b.html#usbswap
Here's a link to a thread in which someone else had a similar problem:
http://www.cableforum.co.uk/board/sh...ad.php?t=12501