What is happening is this. When you transfer data your computer sends out what are called ACK Packets. It lets the computers you are getting data from know you received what was sent. When you saturate your upload sometimes those ACK Packets don’t get sent out. This slows things down since you are basically not allowing the other computer know that you are ready for the the next part of your download.
Most torrent clients allow you to limit your upload. I shoot for about 75% of my max upload. With your connection I would only upload about 25KB per second.
http://i.imgur.com/5YCZ9.png