I'm also on 7.1.1 with the March 5, 2017 security patch. I've never had this issue before. We'll see how it goes today.
It started yesterday with not being able to connect to my car's Bluetooth, so I shut down and restarted the phone. The Bluetooth then connected, and I was streaming a podcast and the phone locked up and restarted by itself. It restarted a couple of more times during the day, not really specific to using any certain app.