I had been wondering about something odd in the
motherboard.
Had not considered wireless interference. I do have two other systems on
separate partitions and recall I did have a similar pasting issue with
Mepis.

Both keyboard and mouse are wired. My amateur radio gear shares the desk,
but nothing was switched on at the time. That equipment runs off a 12 volt
battery system with a transformer trickle charger so noise from a
switch-mode
power supply is not an issue. Thinking further on that thread, there is
only one
transformerless power unit within a few metres, that's for the portable
phone.

More investigation needed. Thanks for pointing me at the possibility of
R.F. interference. Will report progress.