Gorlok-- Do you do any filesharing? Download torrents? Comcast is notorious for traffic shaping, which can mess up pretty much any internet-using programs. From what I've heard their policy is deny, deny, deny. I'll see if I can dig up some sources.

Edit:

Sorry, looks like the original sources have been taken down. But the forum where I first heard this is still up!

http://www.bluelight.ru/vb/showthread.php?t=346544&amp;highlight=comcast
http://www.bluelight.ru/vb/showthread.php?t=350771&amp;highlight=comcast
http://www.bluelight.ru/vb/showthread.php?t=337771&amp;highlight=comcast