There may not be anything you can do... some cable and satellite boxes send video with "extra" pixels, especially on the sides, when there is 4:3 programming and the box is set to 1080i. The only fix is to set the box to 480i for SD programming, then switch it back to 1080i for HD... damn annoying.
When the video display receives the 4:3 program with the "extra" pixels on the sides, some odd geometry things can happen and it can indeed be difficult to get the 4:3 programming to not look distorted.
The HQV chip has a "Through" mode in the Setup menu (one of the choices along with 480i, 480p, 720p, 1080i, and 1080p) - that sends whatever comes from the box without altering it but in this case, it may not do anything for you. It's worth a try, but don't get your hopes too high because what you are seeing is probably a "fight" between the cable box and video display. But you never know... Through might just fix your problem depending on how it is set now.