This sounds like the issue I'm having with two of my family phones. I have a thread below.
After a looong chat with a senior advisor it turns out Apple is looking into this as a software issue. He had started talking me through a full reset as new (had previously done reset all settings and restore from backup), and then said that devices are unexpectedly freezing and restarting. I asked him about ios 10.2.1 possibly fixing this, and that apparently was the right question.
"There is an issue that we are investigating actually. Testing as new is part of the solution but our engineers are hard at work figuring out the best way to solve this for everyone affected. Updates like you were talking about before are most likely going to be how that works. looking at your diagnostic test there is nothing physically wrong with your device and replacing it is not guaranteed to fix the issue. This is purely a software problem at this point."
So, I didn't do the full reset. Several of my daughters' friends are going through this and they all have Snapchat.
From what I remember during a screen share (I couldn't save it) he said it almost sounds like an app is doing something it shouldn't, and affecting the rest of the phone. I told him my daughter will press her home button sometimes and it will freeze, and I mentioned Snapchat as the common factor. It's possible Snapchat is doing something (can't remember the technical details) that affects how the phone handles any requests. Not sure if the blame lies with Snapchat or how the ios handles whatever Snapchat is throwing at it or if it's not Snapchat at all or something else.
https://forums.macrumors.com/threads/anyones-phone-keep-restarting-on-ios-10-2.2021423/
