One of the the most obvious problems, in the current batch, was the propensity of Firefox to crash when I tried to open another tab.
Since my last post, I've uninstalled and re-installed FF and there is a definite performance improvement. It's much faster and doesn't crash. So far.
Prior to that I had a look at task manager and nothing untoward was spotted so I don't really know what was going on.
I'm going to sign off in a minute or two, make a cup of tea for us both, then boot up again. That will be the acid test.