I had a problem with similar (but not identical) symptoms, albeit with a R9 280x.
After crunching GFN22 for several years 24x7, one day it experienced a fan failure (1/2) and caused a thermal shutdown. After that it would act normally just after boot up, and throttle to 300mhz core after several hours (initially) and several minutes (eventually).
This 280x has a secondary heat sink over its VRMs and memory. Cleaning the heck out of ot (when the fan failed whatever oil was in the bearings founds its way to the PCB), and replacing those thermal pads fixed it. It is back happily crunching. With a slight o/c now too.
Personally, 80C is a little higher than I want to see my GPUs. Can you set a fan profile where it doesn't exceed 70C?