MacBook Pro random shutdowns , cause -79

I would backup your stuff using TimeMachine, then using internet recovery reformat your drive and install a fresh copy of your OS.

You might find creating a bootable OS installer thumb drive a bit better if you are using a newer OS elease as the internet recovery often won't get you to the newer release directly.

Download the OS installer from the App Store or from here How to get old versions of macOS format your thumb drive with GUID and a journaled file system and follow this guide How to create a bootable installer for macOS

Then using it as your boot drive use the startup key Option (⌥) to get to the Startup Manager to select your external drive. Reformat the drive and re-install the OS. Once done install your apps if you can from the original sources so you get the newest versions and then recover the rest from your backup as needed.

Reference: Mac startup key combinations