This kind of stuttering implies that there’s either some network issue or that the server isn’t powerful enough for some required transcoding.
Would you mind sharing some additional details on your setup and what you’re playing?
https://support.plex.tv/articles/200871837-status-and-dashboard/
https://support.plex.tv/articles/201998867-investigate-media-information-and-formats/
The recent update to Plex on iOS has changed the default player to one that was previously only available for testing. That being said… in general that should provide a more stable playback experience as the new player supports direct playback of more formats
