Reviewing the recommended versions.
At least for 8.1.X, the version recommended by TAC is 8.1.24.
The recommendation would be to apply an Upgrade and validate if after this, the reboots still occur.
License expiration on a physical firewall wouldn't cause the firewall to restart regardless, that isn't how PAN-OS handles license expiration on hardware. I'm assuming that you've actually been running 8.1.23 for a bit without issues since 8.1.24 was released a few months ago correct?
If you have support on the device I'd recommend opening a TAC case to actually look at the logs to try and identify why it's restarting, but it's quite possible with a PA-200 that you're running into some hardware component fault. You can try upgrading the firewall, but I wouldn't personally jump to that when you have a randomly rebooting piece of hardware. The possibility that it reboots during the upgrade process and corrupts itself isn't zero.
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