In order to help you with your problem you're going to have to supply more information than what you did:
When you say "It's all video from different network channels" does that mean that all videos from the Xfinity website have the same choppy / stuttering / pixalating problem?
Have you tried to go directly to the networks own website to try and watch from there, i.e. any CBS show NCIS, Elementary, Criminal Minds, Blue Bloods, the Mentalist etc go right to the CBS website and try and watch from there. When you try this, do you still have the same issues?
You say that you're using a laptop, how are you attaching to your modem i.e. by wire / ethernet or by wireless? If you are attached by wireless than try attaching by wire and see if there's any difference. Do you have a router and if so what brand and model router do you have?
If you still have the same problems watching from the networks own site and you are attached by wire to a router then try bypassing the router and go stright to the modem. If that fixes the problem then your router may be bad, try replacing it. If bypassing the router doesn't fix it then maybe your modem is bad, try taking your modem to a Comcast store and swap it for a new one. (especially if it's one of the older RCA ones)
If none of that fixes the problem then ctherr9804k's suggestion is right and you need to have a Comcast tech come and check that you're getting good signal strength on your line.
- Norm