When battery gauge fluctuates sometimes car will shut off

It sounds like a couple different things. Since the WP was replaced, that leads me to think there is a loose belt causing the alternator gauge to move around. Do me a favor. I need you to check the alternator output. It's really easy to do. All you need is a multi meter or volt meter. Here is a link that explains how it is done:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Here is a link explaining how to use a volt meter:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

Next, when it refuses to restart, I need you to check if there is spark to the plugs. The crankshaft position sensor may have been damaged when the work was done. However, that shouldn't cause the gauge issue. Do this and let me know what you find.

https://www.2carpros.com/articles/how-to-test-an-ignition-system

One last thing. When the engine is off, check to see if the drive belt running the alternator is tight.

Let me know what you find.

Joe