Problems with uploading

Last month, people using Comcast cable internet were reporting problems with their upload speed. As it turns out, Comcast is deliberately stalling, “delaying”, the upload signal from their subscribers computers, when certain programs were used.
If you are having trouble using your webcam, your Voice over Internet Protocol (VOIP) phone, like Vonage or Comcast Digital Phone, or are experiencing more than usual lag with online video games, then this is the reason.

Published interviews with Comcast personnel indicate that calling and complaining will do no good, because the call center staff have been told to lie to callers.

Read more about it here and here.