I would back up everything to itunes or the cloud and then restore it to factor default.
Then use it for a few hours to see how it reacts, if it works fine then restore from the backup.
This way you can determine if it's a hardware issue and if it's an issue with some app that you have installed.
Checking in to see if you still need help with this issue, if so please reply at your convenience.
Hello there! You can follow the steps below to fix your iPhone.
Sounds good :)