Hmmm... so, if it boots fine without the cable modem installed, then I assume that it's a USB connection that's making it do that -- most likely, the cable modem.
Most cable modems come with both a USB and Ethernet ports on the back, and the Ethernet port is the preferred port to use with your Macintosh. Does your cable modem have both these ports? If not, perhaps you should call your cable modem company and see if they'll exchange your modem for one with ethernet ports.