The misinformation is amazing.
First of all, if you dont replace thermal paste, you wont get anywhere.
Secondly your fan is underperforming cause its "tired".
Thirdly, just because you "see" 120 FPS on the screen, on a 60Hz monitor is absolutely fucking pointless, its literally in your brain that you see any difference between 200 fps, 120 FPS, and 80 FPS on a 60Hz monitor, there is a reason 144Hz monitors are so famous the last 2 years with CS:GO picking up crowd and OW releasing., i would know , i have the same brain malfunction as you do about it, until i played at 144Hz and now i can tell apart staff better on bad 60Hz screens like i own at home.
Fourthly, only reason to have Vsync off or uncapped FPS is when the game is shitty programmed and it bugs the fuck out, Overwatch isnt one of those games.
Fifth, the fact that you have that much FPS is what overheats your card, your tired fan/tired card/non-renewed thermal paste cant handle the power required to keep the FPS that high.
Sixth, GPUs have been burnt over years for the same reason, idiotic companies with uncapped FPS on log in screens and similar shits, forcing cards to try and produce 500 FPS, eventually fucking them up, its what you are experiencing only difference is, you are experiencing it because your card is -tired out-.
In other words, go buy a new fan for your GPU, replace it, obviously new thermal paste and continue using the card otherwise, try using Vsync already.
https://www.arctic.ac/eu_en/accelero-mono-plus.html
This one basically, not even sure why its for 54e, bought it for less 3 times when i had to replace a 2x 560s and a 460.