same thing happened to me on my oldass geforce 6200 after a couple months, it would go over 100+ ºC, i just put a fan on top of it and a fan below it and a fan on the outside to take that shit hot air out. now it's fine

fans are rather cheap and still effective. but if you have some money to throw in you might look into liquid cooling.
ps: i never got WHY it started to heat up that much, like you the comp was completely idle