Hi Sylvia,
thank you for the answer.
I was almost ready to confirm NUC was working properly with "minimal" set of drivers (LAN, USB3, WiFi&BT and Rapid storage driver), but during last 15 minutes it again froze with screen corruption . After I manually switched it off, then switched back on and booted Windows, it was fine for a few minutes, and then the screen just went blank and to power save mode. So I had to force poweroff again.
I will try what you suggested, and post back.
EDIT: I just noticed that Windows update has installed the graphics driver for HD5000 - somehow that got updated with other updates (it was done today after I originally posted this reply). I have now uninstalled Intel driver and reverted back to "standard VGA controller". And so far it looks like the situation has improved - which makes the video driver very suspicious...
Thanks again!