Intermittent black screen pop-up

[Harry_Beast]

This only happened after upgrading to WIN10 from WIN7.I first noticed it when playing LOTRO.The game screen would suddenly switch to desktop,the game itself did not dump-just had to click icon to get display back.I now have to run the game in windowed mode.Now there is a slight hesitation and a black screen with a white heading momentarily flashes,There is some inscription on this screen but it flashes too fast to read it.
This happens also with no games running on the desktop screen--so it is not game related.