Multi-screen - mouse clicks revert to single screen

I'm running triple-screen (nvision), but when I first start a flight and for an indefinite period, any mouse clicks on the screen (eg to program GPS or flick a switch) cause FSX to go black, revert to single screen and pause, which is quite annoying... At some point in the flight that sensitivity goes away and I can click all I like.

Any ideas what might be causing this?

(P3D works fine...)

Cheers

Keith