Hey John,
That sounds like a tricky one. I had a similar issue a while back with my old Samsung TV. It turned out the transcoder settings for that specific device profile were too aggressive. Have you tried going into the Emby dashboard under Dashboard -> Playback -> Transcoding and checked the 'Limit remote stream bitrate' and 'Allow hardware acceleration for remote streams' for the specific client device profile?
Sometimes, it's worth trying to explicitly set a lower bitrate limit for those problematic devices.
Good luck!
- Jane