Hi bob3160 - thanks for getting involved.
I was/am running the latest version of Avast - I'm pretty meticulous about that (but without jumping on beta releases) - and it is a problem.
I spoke with support about this two weeks ago, and they eventually told me it was a 'known Windows issue' which they could fix for me if I took out an annual contract. The upsell seemed to be all they were interested in telling me. It was at that point that I risked uninstalling and voila, manifestation of the problem went away.
I called them again today to let them know that uninstalling seemingly removed the problem, which suggested a strong association with Avast itself. When I discussed this thread with the operator, he said that as a member of staff (ENDT) had got involved in the thread, the likelihood was that there was a fix currently in process for the next release.
I don't really see how raising a duplicate thread on a subject covered here - a thread which is still open and unresolved - is helpful.
My comment here was to encourage Avast to chime in and say "Yep. We've identified the problem and are working on a fix, hopefully for release by X date."