I was called away from desk for a few hours, and left the troubled computer on,with battery removed,and powered from a/c brick.
When I returned, everything was working normally.No error messages.
For time being, problem seems to have cleared while testing, but time will tell if it occurs again.
I have made notes on the offered advice and will try these if the problem occurs again.
I will keep this open for a few days just in case.
Thanks for all of your valuable time and effort on this problem.