Colored Dots on iMac and then system freeze

GPU failure was pretty common with that model. Apple even ran an extended warranty program on it for a few years, but that program elapsed many many years ago. Sorry to be the bearer of bad news, but it is over 9 years old and it's probably best to start looking for a replacement system.

Have you blown the dust out of the intakes lately?

What temperatures does this App report? Especially the GPU.

This App can be used to speed up the fans, all my iMacs would be dead without it.

Never ever ever drop fan speeds below the speed that the Mac itself is calling for.

MacFansControl…

https://www.crystalidea.com/macs-fan-control

Yes, GPU failure due to heat is an iMac specialty. :(

What ever speeds are needed to keep the GPU at least as cool as possible, on my 2001 iMacs the ODD fan has the most cooling exact on the GPU, though speed the others too because keeping other innards cool helps the GPU.

Also, iMacs are Dust magnets.

30°F drop is good, but the longer it's overheated the less heat it takes to kill it off, so use as much fan as your ears can take. :)

Very pleased I could help, not much time left... stay well! :)