Reinstalling OS Monterey via Recovery does not permanently solve my problem

A month or so ago my iMac suddenly spat the dummy and when I woke it up I had the black screen with the circle and diagonal line through it.

The url given (can't remember the exact url but it ended .../startup) suggested that there was a corrupt file.

I went through the "repair" option after rebooting with cmd+R but that didn't solve anything, so I reinstalled OS Monterey. Once the OS was reinstalled everything was OK for about a month - then again I woke the mac up one morning and there again was the black screen.

To cut to the chase, I've now done this twice with the same result. The Mac works great for a few weeks then I get the black screen again. Can anyone please advise me what I should do next?

Thanks.