It could also be a backlight going out, or the control board.
I can't say for sure though, just going from my guesstimations from work, I replace a lot of TV's at work and some exhibit the same behavior and the running theory amongst the other techs are what I mentioned above. We don't actually fix them though, we just replace them, and send them to our bench techs who possibly just trash them, because I've never seen a fixed TV come back, they just send new ones.