Well, doing a bit more reading and all the answers seem to be pointing to either booting in safe mode and trying to figure out what is running, and when, and then to keep disabling things to see if the problem goes away, or...................... the quick way.
Saying "to Hell with it" and doing a factory reset - a bit like a fresh install of Windows when nothing seems to be working! There are details
here with instructions on backing up your personal data.
It does look like too many things all trying to do things at the same time and using up CPU cycles as well as loading up your RAM with junk.
There are times when even people like me go the 'sod it!' route. We save as much personal stuff as we can then go back to a pristine OS and start again.