Thank you. I didn't give a full enough background in my 1st post. The shutdowns have happened maybe half a dozen times over the last 3-4 months. After the 3rd or 4th time, I booted into Recovery mode and did 1st aid on the internal drive - no problems reported but quite a while elapsed before next shutdown, and then another after a couple of weeks.
I found this page which outlined what to do if software or hardware - hence my original post.
I'm going through my Applications folder & deleting old 3rd party software. Hesitant about disconnecting 2 external hard drives (did 1st aid on these while in R mode - no issues reported) because one is ™ and the other for daily internal drive backups with Carbon Copy Cloner. Only other peripheral is quite old Brother laser printer and am keeping that disconnected except for occasional use.
If I get another shutdown with a different report from the one just posted, I'll be glad of your help again.