If you're talking about a pattern of pastel coloured short lines all over the screen, it usually is the video card memory. It may be heat related, or dust, or both. Here in Finland we're suffering from a subtropical heat wave with 80+% humidity. Such conditions can make accumulated dust to gather moist and cause shortcuts here and there. Un- and replug everything, cleaning in between with bottled air. Be cautious for static and don't let plugged fans rotate with the air blast.
It might be something else as well but cleaning and reseating never hurts.