I described this exact condition in another post. Photo here:
http://audiokarma.org/forums/attachm...0&d=1162180974
With the lighter zone (from the girl's collarbone to her cheekbone) starting at the bottom and slowly walking up the screen, just as you describe. I attributed it to the Ramsey VHF broadcaster kit I was using, as all of the TVs in my house were doing the same thing, when tuned to the Ramsey's signal. I never solved it, but knew it wasn't a problem with the set I was working on, so I didn't worry about it.
Hopefully we'll get a good technical explanation...? I'd like to make the problem go away too.