Some programs are HD but broadcast in 4:3. ESPNHD often does this. I don't have comcast cable so I can't comment on them specifically but directtv and my HDTV have a "Native" mode that displays the picture as it is broadcast. So a 16:9 picture originated image is displayed in 16:9 format on my tv, an image originating in 4:3 format is displayed as 4:3 on my tv. In Native mode I never experience stretched/distoted or flattened/cut off images. I always see the image as broadcast, whether HD or standard and in the correct format.
This should help. In the event it does not, the author assumes no responsibility and is held harmless from this post.
Using the cable remote, turn the cable box off using the remote's POWER button.
Press and hold the red MENU button on the remote until the cable signal returns and a blue dialogue box apears on the TV screen
Select TV ASPECT RATIO using the up and down navigation in the middle of the remote
Set the aspect 16:9
Exit the display by pressing the POWER button
Turn the cable box on using the POWER button
Check and you should the have the correct picture.
Tech support may keep you on-line all evening trying to correct this issue.
I hope it helps with your problem.
I hope it helps.