iMac Freezes then Restarts

I am having an ongoing problem with my iMac freezing and then restarting.

My iMac is a Retina 4K, 21.5” late-2015 with 8GB HDD running Monterey 12.7.4

The machine freezes without warning. There is no rotating “beachball” and there is no error message. After a period of around 30 seconds the screen goes blank and then the machine restarts. The restart is slower than usual, suggesting to me that the operating system might be sorting itself out. Occasionally, the machine reboots again during the restart.

This problem does not seem to be related to using any particular application, but most often is when working in Safari or on a Zoom or Skype call. Once or twice it has even crashed while the machine was standing idle.

I have run the built-in diagnostics numerous times, I have reinstalled Monterey four times, twice doing a complete clean install on a reformatted hard drive. I have run MemTest86+, Onyx and EntreCheck, without any problems being detected. There are no crash reports in Console. 

I really don’t know what to try next. Any guidance?