There's a thread with other people complaining about the same thing here http://forums.comcast.com/t5/XfinityTV-Website/Video-Freezes-amp-Will-Resume-Shortly/m-p/1651149#M23691
Several suggestions: try and watch directly from the networks own web site, are there any problems doing that or does playback work fine?
Check that your Flash version is the latest http://www.adobe.com/software/flash/about/
Try this https://forums.adobe.com/thread/695456
Try and use the Adobe Flash uninstaller http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html to uninstall and reinstall Flash.
If you are using Internet Explorer, try FireFox or Chrome.
If all else fails send a private message to one of the Comcast support people; provide as much information about your problem as possible; what's shows you're having problems with; that streaming works from the network site but fails on the Xfinity site (or not); what browsers you've tried; what version of Flash you're using; every piece of data could help point to the problem.
- Norm