If this was my phone @bod69 I'd back up and factory reset but when resetting back up don't actually use the back up for now.
Set it up as a new device and then see what happens.
If it stops then use the back up and see what happens.
Don't use Smartswitch.
If it continues then the back up maybe corrupted.
You could also try > Safe Mode > https://www.samsung.com/uk/support/mobile-devices/how-to-start-my-galaxy-device-in-safe-mode/ turns off downloaded apps and themes etc which may highlight an app or theme that's causing conflict.
If it continues as a new set up then I'd suggest to have the phone looked over via a Samsung Service Centre or if in your cooling off period have it swapped for a new one.
I wish you all the best. 
@bod69: The frequency of the restarts is a bit of a concern, and suggests that it is probably not an automated feature. To double check, please head to Settings > Battery and device care > Tap the 3 dots in the top right > Automation > Auto restart at set times, and disable this option if it's active. Tap the Back button on your Navigation Bar, then select the graph symbol in the top right corner, next to the 3 dots, to check your restart history to see your last auto restart.
If the above isn't applicable, I recommend uninstalling any recently added third party apps, just in case the problem is stemming from an external source. You can also head back to Settings > Battery and device care > Device protection > Scan phone, to check for malware and suspicious activity.
