Me and my SO have two 900c 65" Sony TVs. When watching youtube videos directly through the native app on his TV, we get constant stuttering on the tv on any quality selection, but the audio is fine and smooth. If I go on my phone and through the youtube app CAST it to the TV youtube app, instantly smooth. Highest quality, no problem. It only has issues with the native app on the tv and using that directly. Our other identical TV? No such problem, same internet connection, and both tvs are updated to the latest update.
Is this common? Anything I can do to try to fix it?
No issues on my Sony Android TV. You could try factory resetting it and then updating/installing all your apps again.