I have the same issue with mine, except mine is the right third of the screen. The picture will be fine for about 10 seconds after start up, then it goes to the vertical lines/black and back and forth.
I found somewhere else that it means the LCD panel itself is going bad, but my assumption would be that if the panel were going out, it would be the whole panel, not just part of it. But I'm new to the tv repair scene. Any insight would be great!