What IP range are you using on your LAN? If your modem's diagnostic interface falls within that subnet, you're bound to have connection issues with it as the router typically doesn't route LAN traffic through the WAN interface.
I had similar issues in the past and resolved it by changing my LAN IP range so it doesn't include the modem's diagnostic interface.
@kruser: Thanks! Hadn't thought of that and will try it tomorrow morning, if I can remember to get on the computer that is closest and easiest to mess with wires.
@passerby: My LAN's traffic is in the 192.168.1.2 - 192.168.1.113 range, set statically and always has been. -- The Firefox alternative. »www.mozilla.org/projects/seamonkey/
