i have noticed when i try to actually use the screen to focus... the screen almost always goes blank for period of time... then comes back on... then off - on off -

what causes this - it is very frustrating

I've had the screen blinking on and off, even when I am not close to it.  The culprit was a dusty eye sensor which made the camera think that I wanted to use the EVF.  I cleaned the sensor and the problem went away.

Or as happens to me occasionally with my A7r3 the screen blacks out because I am holding the camera too close to my body or some other obstruction that leads the eye sensor to think you are holding the camera to your eye.