Have you tried this in safe mode to see if it happens when 3rd party apps are disabled? If it stops in safe mode it will be a 3rd party app causing it.
The process for turning on Safe mode is as follows:
Press and hold the Power button.
Tap and hold Power off.
When the Reboot to safe modeprompt appears, tap again or tap OK.
Your device will then restart and it will say “Safe mode” in the bottom-left corner.