Tell me about your work with canned air on the vents and if you do your own repairs. That is, would you spend 20 bucks for canned air, heatsink compound, maybe some tool to get in there and watch YouTubes? If not, just get a shop to do a deep clean for you.
iMac is 10 years old in a high desert environment and for the past few years it has overheated. It has now reached the point where it has closed down from overheating.