Thanks. I have thought it a hardware issue - the fact that it’s been intermittent and doesn’t seem to affect many other devices. I’m not sure how/when the tapping takes place, but just tried it, or a form of it, and it didn’t make any difference. I wonder if there’s a way to prove that’s the cause, and I’ll just know I have a broken device?
It would be nice to know one way or another, so I know it’s not a race condition or similar in the OS.