Are both the TV and the cable box set to 16x9?
Often times, some cable boxes may have an AUTO setting for resolutions. Pick 720p or 1080i as the output resolution in the cable box and see if that helps.
My PN50C550 is having trouble adjusting the picture size to the type of signal it receives from my Scientific Atlanta Explorer 4240HDC cable box. When I first turn the TV on, its generally fine. But then when I switch from a 1080i channel to a 720p channel, or vice versa, or from HDTV to SDTV, the TV gets confused and zooms and offsets the picture in a way not suited for the signal thats coming in.....and part of the picture is cut off either on the bottom or right, or both. My Panasonic S2 1080P HDTV handles the signals from the same cable box through same cords and from same output just fine.
Is this a known Samsung issue? Samsung support online and on phone was no help. They want to send someone out.