If the issue is not being caused by an app, check for updates to your phone's software and if an update is available, download and install it.
To prevent the phone from powering off due to high temperatures:
For phones with Android 5 Lollipop or higher, Device Protection may be enabled, which means that after the factory reset, you'll need to sign in using the same Google Account that was logged into your phone. If you forgot your Google Account password, use your computer web browser and go to www.google.com/accounts/recovery to reset your password first.
To perform a factory reset:
If you have a security lock on the device, you'll need to enter your security information as well.
The phone will reboot and reset.