he got a used modem. so the mac address was already in the comcast system and they never removed it. I had it happen when i was on adelphia. They will catch on eventually. But then yet its been 6 months and i still have free cable tv so the cable companies aren't the smartest tack in the book.
J_E_R_T try tweaking with TCP optimazer with the setting 7.5mbps. also try this test but clear your cache before you test http://www.testmy.net/tools/test/d_load.php?step1_timed=&step1_size=&step1_validation=&testall=&s=5983&st=st&ni=ni&ta=&top=&tt=1&nobar=