Have u tried resetting the cable modem? I have a surf board cable modem and if I goto
http://192.168.100.1 I have access to the cable configuration, you can reset it there and clear all current MAC Address stored, probley will be down for a while, also if you upto and know, great way to spoof a MAC addy, ARP posioning anyone??(thats your dark tip for today)
BTW, have u scanned for spyware lately?