There have been many similar complaints; there's a thread specifically for advertising related issues here http://forums.comcast.com/t5/XfinityTV-Website/Ad-Issue-during-Video-Playback-on-http-xfinitytv-comc... and you can read the recent reply by ComcastMatt that these issues have been reported and are now in the process of being addressed.
If you feel that you want to add any additional information then the best thing is for you to post the details there.
- Norm