Solution here:
OfficeBackgroundTaskHandlerRegistration flashes a window every hour - Microsoft Community

Thanks Abibi.

Before seeing the above link...I used a pocket camera to video the window flashing...brought it in to a video editor and saved the frame...I could barely read it, but it was obvious...

There's another post here about this abibee...have a look, there's some helpful info there:
Quick Window Flashing / Popping Up Periodically - Windows 10 Forums

I'm having a similar issue.
When i'm in a game, full screen...the displays go black for 3 seconds...then the game is in Windowed Mode.
If i'm not in a game, I see the outline of a Windows "window"...