iMac shuts down randomly and suddenly

@danj I don't think it's something related on the CPU. I had this idea before you but then I tried to stress it for multiple minutes and got no shutdowns. Yesterday the system ran stable for 6 hours without running anything which requires graphics acceleration. If you open anything that utilizes the GPU much like a YouTube video with 1080p, it goes off within 10 minutes. When I delete the graphics driver in Windows or start the Safe Mode on macOS, the shutdowns are completely gone. So, finally it must be a GPU issue although this shutdown behavior is very strange for a GPU error. I ordered a new GT 130M MXM graphics and will try it in the next days.

Well it would be a surprise if it was the MXM GPU. Let us know what you find out.

In installed the GT130M (from an Acer) yesterday and unfortunately it does not work.

The system boots up but the screen stays black and connected via VPN from another computer, I figured out that the GPU is not even recognized by macOS.

If I insert the original GPU again the iMac boots up normally. So I have to go for a genuine Apple MXM GPU.

Problem is, that these are hard to find and mostly expensive. Also I don't know which GPUs will work.

I don't want to go with the slow GT120 again and an Apple custom GT130 is not available.

This series offered two MXM GPU's:

661-4990 - MXM III, M76XT+/256MB GDDR3 (NVIDIA GeForce GT130)

661-4991 - MXM III, G92GT+/512MB GDDR3 (NVIDIA GeForce GT120)

And yes, the Apple boards have custom firmware chip which is why the Acer board didn't work. Look on the Web do see if someone offers the programed chip so you can replace it.

I decided now to go with a new logic board containing the 9400M chipset graphics for $50. These graphics are all slow in today's circumstances and it does not really matter for me if I have a GT130 or 9400M in my iMac... Additionally, I avoid the risk that the logic board could be faulty in the end after all the expenses for a new MXM graphics.