audio out of sync using xfinitystream app on firestick

Audio and video are constantly out of sync. It's hard to watch when it's a little out of sync; impossible to watch when the difference routinely carries dialogue into the next scene.

This is an ongoing problem. It's an issue with recorded shows and with on demand.

This is not an issue with any other app, or with live TV.

Everything up to date. Uninstalled and reinstalled the app a dozen times or more.

What can be done?

Thanks.