If it can't establish an IP address, that means it can't contact the DHCP server, or all the IPs allowed for the modem are in use (on Road Runner, you have to power down the modem for a few seconds in order for it to free up IPs, can't say for Comcast). If you only have one IP, and you're switching between a computer and your XBox without powering down the modem, that may be your problem. Otherwise, if the link light on the XBox side is on, I'd expect something is wrong on the other end of the modem.