Sorry DaleB to hear you're having these problems. Know how it feels. :o(

My experience may not be even close to what is causing your problems but I figured I share them anyhow.
The last "issue" I encountered started with the computer staggering and sometimes even slow down to a crawl.
Not always, not with same applications running and not with a noticeable "pattern". And it got worse and worse within a month.
When doing a monthly backup of my work disk and zipping the whole thing I happened upon the problem.
At a certain area/directory/files the zipping slowed to a crawl.
I unplugged the drive and everything worked fine.
I never got any disk problem report when doing disk checks.
Anyhow, I was able to get everything moved to another disk.
Formatted the problem disk and loaded it with test stuff with no problems. Am using it as a storage disk now.
Haven't got any problems with it or the rest of the computer since.

I still don't know what the actual cause was but I'm happy it works. :o)