I've seen this on 550's thru 750's.
Remove the "Just Scan" feature for the resolution. I would rather select 16x9 or 1080P whichever applies to your model. But in any event unselecting "Just Scan" will solve this problem.
Hi, when watching tv i notice sometimes these white flicking i guess bars at the top of the screen. They do not always appear and when they do appear, it only happens on the SD channels or HD channels that are not full screen. It happens only on the top and ony where the tv picture ends and the black filler bar begins. Im guessing it is all apart of crappy comcast signal but if you could confirm this i would appreciate it. I have a 52650 and any other info you need i will gladly provide.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic