Update report.
I'm a bit further forward.
Removed the cpu cooler and the cpu yesterday. The thermal grease did
not look good. It had obviously been disturbed when I installed them
and was not evenly applied. Removed and applied fresh, re-assembled
and it's run ok since, with no random crashes since. I'm guessing this
was aggravated by the increase in ambient temperature.
I put the original power supply back in, too.
But the system was still running slowly. I noticed that OneDrive was
using a lot of cpu cycles so closed it down. (My data is stored on a
NAS and some it is backed up to OneDrive so I don't know what it was
doing, or trying to do, on the PC.) There was some slight improvement
in operating speed and booting was quicker.
Then I noticed that Windows Problem Reporting was also active at the
top of the list of cpu use so I've turned that off and noticed more
improvement.
Ran MalwareBytes and no issues reported.
The fan resistors that I mentioned are intended to slow the fans down,
to make them quieter. Taking them out I haven't noticed any difference
in fan noise. (And I can't tell you what difference they made to the
speed.)
There's still a delay in switching between programmes but nowhere near
as bad as it was. As I type this task manager reports that the cpu is
running from 5 to 15% capacity and the ram at 15% (with Outlook, Edge
(three tabs) and Agent running) so I don't see any obvious reason for
the delay.
The OS is on a 500Gb NVMe SSD with 300Gb free so I don't see any
explanation there, either.
Windows has just shut down the ethernet connection with a 'Windows has
stopped this device because it has reported problems. (Code 43)'
message in the device manager. Removing it, deleting drivers, and
scanning for new hardware has brought it back.
We'll see what happens next.