iPhone 8 music cuts out when using bluetooth with my car audio system.

Another post suggested that Resetting your Network Settings might fix the issue for you: Settings>General>Reset>Reset Network Settings. You will have to turn on Wifi again, and may have to re-pair your device in the car, but see if that works for you as it did for the other poster.

Cheers,

GB

Is it happening when you get an email, message, or notification?

If so, it's just what happens. It doesn't matter if you are using Bluetooth or not.

Cheers,

GB