Since upgrading to Sonoma, multiple unanticipated restarts

You may have some incompatible hardware, or a hardware that just happened to be tickled by how Sonoma may be addressing memory differently.

Use Apple Diagnostics to test your Mac - Apple Support

If any errors are reported there, you have a hardware issue that needs to be addressed by Apple directly.

Otherwise, look at this tip to isolate background tools which may have become redundant or unnecessary or incompatible with Sonoma:

Etrecheck - the handy reporting tool for … - Apple Community

I realize it is difficult to operate in Safe Mode, but while in Safe Mode, did it restart? Did you spend enough time working in Safe Mode to determine that? If it doesn't restart while in Safe Mode, that implies some system modification you have installed is not compatible.

The EtreCheck report would indicate if you have anything installed which might cause problems.

If it isn't a system modification, you might benefit from reinstalling macOS.

Etrecheck found some major problems, but when I looked at them they were marked as 'probably okay' by the scan. Most of them had to do with long-ago programs, tiny little bits that didn't get deleted with the rest of the program. I went through using Etrecheck and deleted what it deemed unnecessary, and ran Etrecheck again -- I went through this process three times. It's a little confusing because even after the third pass Etrecheck tells me there is a major problem, but I can't identify it from the report.

But I think things are improving. Thanks again.

What you need to look for in the report is if you have any kernel extensions installed. Those are what cause it to restart. If you have no kernel extensions, then it is either hardware or some issue with Sonoma and your Mac.

I've tried to reply but keep getting a message 'Apple can't save your message at this time.' So I'll try to send this first.

It does that from time to time. There is a time limit between posts, but it is something like 30 seconds.

I just got the same error on a post I replied to without having posted anything for at least a few hours.

so I guess that's my next step. I'll let you know what happens.

That depends. Do you have a kernel extensions installed. There is a section for kernel extensions in the report, but it won't be there if you don't have any.