Do you get any error messages, such as an overheat warning? An optimization warning? Have you downloaded any new apps that may be interfering with performance? The phone will shut down and restart if it detects an overheat condition, but it will a) usually slow down or freeze up first and b) warn you before shutting down. A toast may appear saying something to the effect of 'your phone is overheating it needs to shut down'. If you have auto optimization on (settings/device care/auto optimization/auto restart) and have the option to restart when needed, that could also be a cause, but that too will give you a warning before restarting. These warning toasts may be very brief and you could miss seeing them. You can check your notification history to see if there is a history of a toast popping up. Settings/notifications/advanced settings/notification history. To determine if your phone has been overheating and what may have been causing it, you can download Good Guardians and the Thermal Guardian module from the Galaxy store.
Try clearing the phone's cache partition. There may be some leftover temporary files playing havoc.
Update to the latest software available, and try to do factory reset.. but before factory reset.. you could back up all your important data to your computer via Smartswitch and after factory reset done, you can restore your data..
