In most cases, the cable/sat box is set to AUTO adjust either resolution or the aspect ratio. Rather than having the cable box choose each time the channels are changed, you will want to go into the VIDEO menu of your cable box and set it to 1080i as a resolution and 16x9 as the aspect ratio.
If you need assistance with this, your cable provider would be able to help you navigate the menu if you can't find them. However, in most cases, this fixes the problem you're describing.
While the overwhelming majority of cable techs are definitely valuable and intelligent, this is a setting that can be often overlooked. I know it may seem like I'm asking you to do something with a box that you may not think is the problem, but that would be my recommendation based on feedback on the forums.
I recently purchased a Samsung PN50C540G3F and the picture does not display properly when going from a HD channel to a standard definition channel. The picture size on the TV is set at 16:9 which is the recommended setting in the manual.
Basically when the TV is switched from a HD channel to a standard channel the actual picture displayed on the screen overstretches to the right of the TV. For example if I'm watching an interview with 2 individuals having a discussion, the picture is perfect when its on a HD channel. When I stitch to that same channel in standard definition the screen will display the person on the left of the TV (albeit stretched) but it will only display one third of the person on the right of the TV with the balance being cut off (it appears that it is stretched right off the screen).
I had the cable box replaced twice and I had a cable technician in to look at the problem as I thought it was a cable issue. The problem has continued.
What setting should be changed on the TV to correct this problem?