Make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple. Also, see If your iPhone, iPad, or iPod touch unexpectedly restarts.
I would try to install a fresh copy of the latest iOS, restoring your device to factory settings.
Make a full backup of your iP8+, and follow the steps provided in this support guide:
Then set it up as new and, before restoring from your previously made backup, test it.
I you have the same issue, then you could have a hardware problem.
If your device is working fine, then restore from your backup.
If you have the same issue again, there could be some software problems (some wrong settings and/or some apps not working properly).
No it didn’t, it actually started to get worse. I went to the Apple store and they replaced the phone. I had the new update, did a full restore through iTunes on the computer even turned off all notifications and restarted then turned everything on again. They determined it was hardware failure and just gave me a new one.
Question: iPhone 8 Plus crashing/restarting