I have an HDHR cable-card tuner that I use with my Xfinity subscription. We actually don't watch much cable, so I never noticed this phenomenon until this week. While watching CNN live, every now and then the video will freeze but the audio will keep going, and then the video will unfreeze and playback with a "fast forward" effect until it catches up to the audio, and then everything is back to normal.
Note that I have never observed this effect watching a broadcast station live, or when watching any DVR-recorded content (broadcast or cable).