Hi Joe:
I've experienced this problem for almost a year (as well as commercials getting stuck and the episodes never continues) and more often than not with CBS shows in particular, sometimes it will stop for a while and then come back. It's almost certainly some problem on either the Comcast system or the addvertisting servers that Comcast uses. Unfortunatly you won't get any help here from the Comcast people, either they will never answer you or if they do they will simply ask you for some useless system information and again never answer. If you try and call Comcast support, anyone you talk to will deny that there's a problem or that they know anything about it even if you reference threads here with dozens of people complaining about the same problem.
The only advise I can give you is the same advise I was given when I first posted about this problem: Go right to the networks website and watch the episode from there, it's always worked fine for me and I'm sure it'll work for you.
- Norm