Don we have Comcast as well and use their Motorola DCT6412 set top box and haven't had any issues with the HD programming or HD playback through the box. Sounds like it may be a defective tuner, you might want to take your set top box in and swap it out for another. I've taken several boxes back and won't hesitate to do so with any sign whatsoever of the slightest of issue.

Here's the tuner we currently use ... DCT6412 Tuner

Rick
Our Room

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