I would guess (and this is just a guess) that something is mis-adjusted in the built-in scandoubler module. Perhaps the timing is slightly off? I think it has a potentiometer somewhere on the motherboard.
I have a scandoubler card that exhibits a similar problem, only the inverse. The horizontal lines start out a few pixels offset too far right, and then come into place as the card warms up. After maybe a two or three minutes, it displays perfectly. I always just figured if it got any worse, I'd mess with adjusting it, but so far, it's stayed the same.
