http://www.speedtest.net/result/2105661178.png (Unsupported image host)
This is the type of internet I deal with on a daily basis, and there's nothing I can do to fix it. I can't switch ISPs, there's no lines to fix.
You, on the otherhand, are most certainly dealing with a damaged line of SOME sort. Or if it's not that, then a TON of people torrenting off your wifi or something.
Call Comcast and they'll most certainly(even if they put up a bit of a fight) fix the issue eventually.