I have a plan to help all of you. First if you are streaming Wirelessly via your router, I hope you know that WiFi can slow internet speed a great deal. If you can hardwire your computer and check if that helps. From the research I have done I do not think it will help. Click on this Link.
http://forums.comcast.com/t5/XfinityTV-Website/Video-cannot-play/m-p/2115074#M32308