Comcast has long been suspected of "Throttling" which is the progressive slowing of Internet speed as you download files. Starts off fast and then slows to a near stop. Read this article from the New York Times:
http://www.nytimes.com/2008/07/12/technology/12comcast.html?ex=1373860800&en=fb1ecb32c57b72d2&ei=5124&partner=permalink&exprod=permalink