The shapes you're seeing are called artifacts, and they are usually because the card is over heating, or its about to die. If its overheating, first check to make sure the fan on the card is spinning freely while the computer is on. If not you gotta replace, that, might as well go aftermarket. If it is, then you have to get the case ventilated better. Also if you have the card over clocked at all, back it down until it doesn't artifact anymore.
If it keeps doing it no matter what you do, it may be time for a new card.