I am using an iPhone 6, and all of a sudden the screen periodically becomes non-responsive. It happens 2-3 times a week, and there doesn't appear to be any method to the madness. For a period of time ranging from 1-10 minutes the screen is 100% frozen. I can't turn the phone off because I can't swipe the shut-down slider. Eventually it starts working normally, but the inability to answer or make calls or texts or emails is maddening. Any idea what's causing this and what I can do to fix it? I always update my software when updates are available.