First, a little explanation may be needed as to what is happening in between you, Comcast and the Internet. Basically Comcast has been using a packet filtering platform called Sandvine. Sandvine is a essentially a Quality of Service device that actually has legitimate uses, such as giving high priority to Xbox Live communications and VOIP.
Unfortunately, Comcast has decided to use Sandvine (some say illegally) to impersonate you and send a reset packet (known as an RST flag)
http://www.fsckin.com/2007/09/18/the-real-fix-for-comcast-bittorrent-throttling/