I'm guessing it's a bad connection on the printed circuit board for your video card. This is basically not really fixable without replacement. Sometimes warming up the card will temporarily fix the issue because as the plastic expands the connection is restored. It is likely caused by overheating the card in the first place. First, the card heats and the silicon which doesn't soften with the plastic can buckle. When the card cools it separates the silicon path.
So, replacing the card is really the fix. Also, you have to watch out for heat in your device. Overheating can ruin more than your video card.
<Please recognize that this is supposition. Replacing the card is what I would do if it were mine. This is based on the evidence you provided that when you duplicated the screen you saw the same flaw in both.>