Hello LalaLoca.
Thank you for using Apple Support Communities and for sharing these details with us. It sounds like you're experiencing issues with your podcasts. Specifically, you noticed that the speed of the playback changes on its own. We can understand how this would be an issue. Allow us to investigate this issue and assist in any way we can.
To confirm, does this issue only occur when listening to podcasts in your car? We'd like to recommend some troubleshooting steps to see if we can get this issue sorted out. First, let's close the podcast app and restart your iPhone to see if that helps:
Close an app on your iPhone or iPod touch - Apple Support
Restart your iPhone - Apple Support
Also, if you have any pending updates available for your iPhone, it would be best to install the update. Keeping your device fully updated can help resolve various issues and also help with performance. Just in case, the resources below can guide you through the update process:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Update your iPhone, iPad, or iPod touch - Apple Support
We hope this information is useful. Cheers.