Why is my phone shutting down and coming back on

Hi Jonathon,

Have you timed how long it takes between rebooting? If you do and find that it's right about three minutes, that's a strong indication that you're having a sensor issue of some sort on your phone, as the phone scans its sensors every three minutes and will reboot the phone if it sees something it has been programmed to think is an unrecoverable error. That reboot procedure is called a kernel panic and can be a valuable diagnostic tool for figuring out what's wrong with your phone. iFixit has a wiki page dedicated to that specific sort of error.

iPhone Kernel Panics - iFixit

What you want to do next is to read through the guide and follow the instructions to located the newest panic log you can find. Either take a screenshot or copy/paste the first page of the log, then come back here and edit your question to add that information. Once we have the panic log available, we can then use it to diagnose the most likely cause of your phone's reboot issue and tell you how to fix it.

Adding images to an existing question - iFixit Repair Guide

I've done this myself, so I know from experience that the three minutes you have before it reboots is indeed long enough to locate and copy/download the panic log, or just the first page of it.

Good luck, and let us know what you find!