Definitely a heat issue. Are you sure BOTH GPU cores were at 80-85 degrees or just one of them? Because taken into account the temp and GPU load you mentioned, it seems possible that one core is almost at full load while the other one idles. This is just an assumption, ofc. Can't be sure, because it depends a lot on how you're driver is set up, how the game utilizes your GPU cores, where you are taking your temp readings from and how that program handles 2 GPU cores on one PCB, etc.
If actually both GPUs are at a lowly 40% load at the same time with temps being so high, there is either some serious dust built up, the airflow in your case is totally screwed or you need to replace the thermal paste on your GPU.