@zimma71: Further to the question above by @leicesterMan, please try restarting your phone in Safe Mode, as this will disable all third party apps and help to determine if the problem is stemming from an external source. To enter Safe Mode, press and hold the Power button until the 'Power off' prompt appears on the screen then release > Touch and hold Power off until the 'Safe mode' prompt appears then release > To confirm, tap Safe mode.
Monitor the performance of the device for a few hours, depending on the frequency of the restarts. If you find that the issue isn't present in Safe Mode, then please restart the phone as you would normally and remove any recently installed applications. Unfortunately it won't be possible to identify which app is causing the problem, therefore I recommend approaching this via a process of elimination.