How does the power button feel when you press it? Does it feel loose or unresponsive?
I recommend that you make sure that your system and all apps are updated to their latest version. You can do this if you go to: Settings > About phone > Software update. To update your apps, go to: Google Play Store > Tap the 3 lines > My apps & games > Updated all.
I would also recommend that you reboot your device to safe mode to see if this behavior is caused by one of your downloaded apps as these will be temporarily disabled in this mode.