Screen Distorts and Freezes

This is happening due to the GPU chip overheating and having damaged the soldering connecting it to the logic board. Instal istat pro widget and monitor your temperatures and fan speeds. Make sure all your temp sensors are working and clean out all the dust from the heat sink. Clean your fans and make sure they are working properly. Inspect your capacitors and make sure they are not bulging or leaking. I would take out the hard drive and use it with an external enclosure otherwise it might get damaged if it is being used in high temps.

Honestly tho, once the GPU has been damaged then it will stay like that. The only thing you can do is prevent it from getting worse by reducing temperatures. You can fix the soldering by performing a reflow on the chip, it is hard but not impossible to do, I learned about this procedure here on ifixit when my imac had the same problem. Here is my old post:

video graphics and boot issues