lewis619,

For those channels, you might want to go in and make sure that both your HD channels in the Comcast box is set to a widescreen format. Then, check to make sure your television is in the 16x9 format. Often times, the symptoms you describe are merely picture settings on the wrong setting from either the source or the television.

Since it's just happening on HD channels, there may be a special function in the box that controls widescreen HD content, and your best bet to resolve that might be as simple as phoning in the cable company and having one of the techs walk you through the settings.

Keep me posted!

--HDTech