Things were running fine until I got an automatic update which asked for an restart, then the problem reappeared. I think the update that was installed was:
Intel - System - 2108.100.0.1053
But then yesterday I had two optional Intel driver updates:
Intel - SoftwareComponent - 1.41.2021.121
Intel - SoftwareComponent - 2109.0.52.0
I installed them and hurrah! things are back to normal. Tried reboot twice after this and still good.