Have you wiped phones cache partition? 
@stickybun: Please try heading to Settings > Device care > Auto optimisation > Auto restart, and toggle off 'Restart when needed' (if applicable).
Alternatively, please restart 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.
With Safe mode active, monitor the performance of the device for a few hours, based on the frequency that you encounter the issue. If you find that the restarts no longer occur, then please restart your A14 as you would normally and uninstall any apps that you have added to the device since you first encountered the issue. It won't be possible to determine exactly which app is at fault, therefore we recommend approaching this via a process of elimination.
Hi @stickybun Welcome to the Samsung community 
Try the Troubleshooting suggested, just to add to @rogersdave77 's information on wiping the cache partition if you have not have a computer you can also connect to another Mobile Device,TV/Game Console USB.
