In a similar instance that you speak on,, and there is a lot of great info here, however,, have you considered that the cable modem unit is fried,,,
I have rogers, and i went through a 4 day period of pulling hair trying to figure out a similar issue with my PC's at home,,
turns out the Modem was fried and it was dropping the connection every 20 mins,, I had to reboot continuously, both modem and PC,,,
Pain in the A$$,,,,
In the end the cable guy brought me a new modem ,, and for $50 bucks gave me unlimited free cable,, *( heh heh, just kidding,, just reminded myself of CABLEGUY the movie,, when Matt broderick offers money to Jim Carey to "FIX" his cable),,,
cheers,