Try upgrading to windows 10. Even Win 8.1 supported is halted by AMD. Try it and see what happens, theres a bunch of program you can use to disable the auto-updates.( no worries)
If it is still happening try it in a different computer or something..or a PSU will do.
Do you have time to disassemble the GPU again? see if any WD40 has leaked onto the mainboard, because it causes some serious overheating issues. ( on the memory modules or vrm.)
Note that if its damaged you really cant do anything about it. Just AMD being an arse. Gigabyte cooling is really good but on certain cards, since their engineering decision on making 2fan/3fan is questionable.