Some channels just happen to be located at a channel frequency that can pick up RF interference from problems like moisture entering cables due to bad connections, damage to cable outdoors. This will cause the breakup of the signal.
Indoors, you may have a bad connection. Loose connections on equipment, or an end-connector on a cable pulled loose by something moving the cable.
You can get trouble-shooting on the Comcast site, or by contacting them here as you did for help. A Comcast employee here will most likely reply here and ask you to PM them with the personal information needed to help you out.