IPAD Air Black Screen - like black curtain but not

Hello,

I have an IPAD Air 16GB Wifi Silver and I have the following problem.

Recently (possibly has to do with the upgrade to 9.3.1, but I cannot be sure) after playing with IPAD for about 20-30 minutes the screen goes black while I can hear that ipad is still working (i.e playing a video). In addition touch screen is responsive but you cannot see anything. I can recover it if I press the sleep button on and off and asks me to slide to unlock. After the first time this happens then the frequency of black screen is faster and faster, every 5 minutes and then every 2 minutes. If I leave it to cool then I can work for 20-30 minutes and I have the same behaviour again.

I tried hard reset with the buttons,

I tried factory defaults restore with iTunes as a new ipad

I tried enabling black curtain with the three fingers and disabling it

I disabled/ enabled siri

I tried tapping on the back like some users suggested. I turned auto-lock to never. But always the same behaviour.

Can you please help. Has anybody else this behaviour? How can I downgrade to see it i

Best Regards,

Konstantinos