Hi there chrispy1953,
Thank you for posting to Apple Support Communities. We understand you're having an issue with Music app and using CarPlay. Is that correct?
We're happy to help. It sounds like you've done some good troubleshooting to make sure your iPhone is up-to-date.
Are you an Apple Music subscriber? If so, are you streaming music or playing offline content when this happens? If you're not an Apple Music subscriber, is this music purchases from the iTunes Store or it music that was synced to your device?
One way to troubleshoot if this is local music on your device is to remove it and then either re-download it again or re-sync it to your iPhone. This can help isolate if it's an issue with the music in your Music library.
Here are the respective Apple Support articles:
Delete music in the Apple Music app - Apple Support
Redownload music - Apple Support
Sync music between your Mac and iPhone, iPad, or iPod touch - Apple Support
Sync iTunes content on PC with your devices - Apple Support
That should help. Let us know how it goes.
Take care.