Yes Comcast is notorious for screwing with torrent conections...however remember that your cable speed can also vary depending on your local nodes load at any given time....
I'm on charter's cable service and every 4-6 months my line speed starts to slow down at peak times until charter decides to expand the bandwidth at the local node or however they do it....then it's fine for another 4- 6 months until there are too many new subscribers once again....