we have the same TV...and ours has been doing that for a couple of years now. They never could fix it....and we did try. Drives me nuts.
We've learned how to do a Fonzy tap to it to make it all better...usually stops doing it after it warms up. Lame, I know, but the husband made me return the NEW Digital TV I bought last year because it was 2 inches smaller than what we have now...LMAO