Comcast does not throttle first of all. Your connection probably has issues while running torrents because your connection isn't THAT fast and is causing a lot of slow downs because you're maxing out the connection, especially if you're allowing others to download from YOU.
Secondly, YouTube is known to have issues. Their servers simply cannot handle all the traffic they get during peak times and be able to stream videos without buffering.
All that being said, what are your signal levels ?--