Well, the best way to initially diagnose this issue is to hook up at least one different external peripheral and see if the same issue happens with a different input.
Testing with both a different HDMI signal and component signal would be really helpful in narrowing down the source of the problem (no pun indended).
After testing, if it happens with just the cable box on different HDMI inputs, I might suggest having it changed out. If it happens with all the sources, it may be the processing or panel, in which case, we could schedule a repair.
Let's try and eliminate some of the variables we can test before scheduling either a cable box exchange or a television repair.
There's just not enough information to make a proper initial diagnosis with this information.
Keep me posted!
I just got a new LN46A550. I set it up and first connected to the my cable box (Scientific Atlanta Explorer 8300 HD). The signal is 1080i with a HDMI monster cable. The issue I have noticed is a few times each second, the screen seems to...jerk....is the best way I can describe it. It's almost like every pixel on the screen shifts up slightly then immediately back to it's original position. It's very hard to notice with a moving image, but is very noticable when all or part of the screen is static. It doesn't occur with the TV menus, and I haven't had a chance yet to check any other sources. Any help would be greatly appreciated.