iMac 27" (Late 2012) randomly goes into repeated restarts

I realise that my Mac is old but...

Have a recent issue-in the last week-of my iMac Catalina 10.15.7 randomly going into repeated restarts.

Has only happened while using Firefox 110.0.1 and most often when solving a jigsaw on https://thejigsawpuzzles.com/.

I realise that my Mac is old but...

The issue is similar to what occurs in a power outage ie screen goes black, but on restart goes into repeated restarts.

I installed a new power board about 4 months ago...no other issues since.

At no stage has there been an error message.

Tried starting in Safe mode, but it would not progress from dark screen to the Apple screen.

Able to start in Recovery mode, then used Disk Utilities to repair all volumes.

Issue recurred. Disconnected all external drives and restarted in Recovery Mode....repeated disk repair then restarted.

Did an Etrecheck Pro Scan and reviewed Minor Issues and found nothing of concern.

Completed a CleanMyMac X review and followed suggested actions.

After all of this the computer again went into the repeated restarts, but after the last Recovery start has not restarted.

Left the computer in Sleep mode for the last 15 hours and started normally on mouse move.

As soon as I have copied one large file onto an external drive I will try to reinstall Mac OS in recovery mode.

Another issue which may or may not be relevant is that PS 23.5.4 has been running slowly eg pointer goes into spinning mode when I complete a small action...so I wonder if there might be a RAM issue (24GB RAM installed) although diagnostics does not suggest this....or a memorey/cache? issue with Firefox.

Does anyone have have any other suggestions?