I use to have a similar issue where every summer the GPU would heat up but what was happening was that the stock software that controls fan speeds was not configured well enough to suit the temperatures. So for example what would happen is that by default the fan speeds stay at a low speed when the card is cooler (as do most graphics cards). So strictly for an example, the fans would be at 20% speed when the GPU was at 55°c, 30% at 60°c, 40% at 65°c, 60% at 70°c etc. you get the point.

I don't remember the exact temperature but what was happening was that the moment the GPU hit 80°c I would start getting artifacts, tearing and crazy stuff going on like your screenshots. What I found out was that the software by default was configured to only crank up the fan speeds to 100% until 94°c! I looked up the safe max temps for my card at the time and they listed 95°c. So it starting to crud out at 80°c was an unexpected problem.
What I did to fix it was downloaded a program called MSI Afterburner. It allowed me to configure what fan speed will kick in at the temps I tell it to. So I reconfigured it to gradually increase and to kick in at 100% when at 77°c. (It's possible you already have software that came with your motherboard or graphics card that allows you to configure it yourself. I used Afterburner because I've always had great luck with it for configurations and easy overclock adjustments.)

The problem was solved but unfortunately the 100% fan speed was loud as heck! Installed a liquid cooling kit and never looked back =P

Not saying this is your problem, but I wanted to share my experience in case it may be of help to you or anyone's help as we are getting into summer.

Good luck! ^.^