Why does my iMac restart randomly?

My iMac has been restarting randomly for several weeks. The screen jumps to black and the Mac restart with the familiar Apple Start tune. If the iMac starts cold, the chance of a random restart is higher. Once it has been switched on for a while, the machine works normally. But even then it can happen that the iMac reboots suddenly and randomly. In short: you can not trust the machine.

To be sure, I have done a clean reinstallation of High Sierra. No random restart during installation (s) from USB stick.

NVRAM / PRAM reset done. No improvement.

SMC reset done. No improvement.

± 3 years ago I have already provided this iMac with a new VGA card by Apple because of a well-known problem Apple recognized. So I think the chance is small that there is a problem there.

Apple Diagnostic Tool done, normal and comprehensive. No errors were found, not even in the RAM.

It is an iMac 27 '' Mid 2011 EMC 2429 / A1312 with Intel (Quad) Core i7 3,4 GHz, 4x4GB RAM and 250GB Samsung Pro SSD.

My big question: What is the defect that "sometimes / regularly" the restarts arise?

My guess is that the Power Supply needs replacing. However, why do I not suffer from the problem during installation.

On the internet I come across the same issue a lot, but nobody provides a clear solution. Hopefully there is an expert here who has already got experience with this issue.

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