My iPhone 6 keeps getting black screens that won't go away and I can't get rid of. Its almost as if the phone has shut off. What do I do?

I just got my 2nd iPhone 6 and my new and old one had this problem after installing IOS 9. The phone will lag for a split second then go all black, usually when I click on an app. Nothing on my phone will work except the last app I was on will open but I can't do anything but look at it. The only way that has worked to get out of this is to let my phone die and then restart it. Any help on how to prevent this?