iPhone 12 Random Restarts/Reboots followed by Disabled/Greyed-out Wi-Fi and Bluetooth Settings.

My son's iPhone12 is randomly restarting/rebooting both at home on the Wi-Fi and outside using our data carrier. Once restarted, the Wi-Fi and Bluetooth usually won't connect. You can click the buttons, they will turn blue and 'search' but then turn grey again. The Wi-Fi button in settings is then greyed-out. After a number of forced restarts, everything will work until the phone eventually reboots itself again.

We've done the usual. We've tried turning airplane mode on/off. We've reset the network settings. We've gone as far as erasing the phone and reinstalling the iOS. That seemed to work at first, but then the phone restarted shortly thereafter and back to square one.

I'm getting very frustrated. Any suggestions would be appreciated.