Intellivision Game Graphics Problem

I got a used Intellivision at a pawn shop a few weeks ago, and it was working for a while, then the graphics on both of the 2 games I got glitched up, making them unplayable. I switched TVs, and it worked again...for a bit, before glitching again. Now nothing seems to work. I'd appreciate a response that makes sense to someone that has absolutely no technical knowledge of these machines.