My 5s running 8.4.1 is having a peculiar problem; it will shut itself off unpromted. The only way to turn it back on is to hold the power and home button simultaneously for 15-20seconds and then it goes through the full boot cycle. When it comes back on it still has battery life. This started happening when I upgraded to 8.4 and it happens about once a week. The battery life on the phone isn't great but it's still manageable. Any idea what is causing this issue?