Sorry to hear that you're having trouble keeping the tunes going.
Please check out my post regarding the settings for sleeping appshere.
Something else to note, when updating the OS on Android devices, the battery optimization on all apps may be automatically turned back on by default.
This can cause performance issues or interfere with playback when the app is backgrounded. Due to the sheer volume of Android devices, we are unable to provide step by step instructions for each and every device. I recommend looking up the steps to manage battery optimization for the specific make and model of your device and disable this for the Pandora app.