Thank you for your suggestions.
This happens when the pc is in full action, even immediately after I minimized fsx
GPU is functioning without any problem, newest drivers installed and I get super numbers from the Heaven benchmark tool.
My best way around the problem, until now, is: hoover with the mouse over the icon in the taskbar; when the miniscreen appears, put the mouse pointer in there; this results in the fsx full screen appearing again; I then have to hit enter and the program continues.
But I thought computers were made to make life easy...