2020 27-inch iMac continuously shuts down and restarts

I have a 27-inch 2020 iMac, and the MacOS is Sequoia 15.5. It is currently using 217.68 GB of 499.96 GB on the hard drive. 115.68 GB of that is "system data." 282.25 GB is available space. I have 40 GB of memory, the extra 32 GB of which was installed in 2020 at the time of purchase, purchased as recommended by the folks at Apple at the time. My iMac had a few issues shutting down a few years ago, but tech support helped mostly resolve those issues.

The usual native Apple apps are installed along with the usual apps for saving and reading documents (Adobe pdf reader, Instapaper, MS Office Suite (installed long ago), Kindle, but nothing weird or more niche. Applications take up 26.47 GB of the hard drive space.

Now, recently (within the last few months), my Mac has been randomly shutting down, sometimes restarting, in the middle of a project. I am a lawyer and I create lots of Word documents for work. I frequently have a legal research site open and I cut and paste text from the research site to my Word document or to an email message in Outlook. I have stopped using my iMac for this because of the frequent shutdowns--it is a total pain to lose work and have to re-do things, I try to save often, but invariably I lose some amount of work. I do most of my work on a work-assigned laptop, but for intensive reading I use my iMac because the large screen size makes it so much easier to read dense legal texts and to draft documents. Other than that, my usage is not intensive: attending or creating Zoom meetings, email, social media, photos, reading magazine articles, watching presentations and interviews on YouTube, etc. Most of my documents are stored on an external hard drive.

As a result, I have recently run Malwarebytes and CleanMyMac for both cleanup and virus scanning. Neither finds any issues and both have eliminated some unnecessary junk, but the shutdown/re-start continues to happen. I have also moved any remaining documents from my Macintosh HD to my external hard drive. I have restarted, cleaned, restarted, shut down, restarted, but the random shutdowns continue, sometimes in fairly rapid succession as I am trying to restart. I have reported all error messages to Apple when that prompt pops up.

I have read other similar threads, all more than two years old, and have found nothing more recent that discusses my issue. I am really hoping I don't need to replace my Mac. Except for this random-shutdown issue, it has served me well.

I would appreciate any advice.

LKH