Something of notice has just happened. I’ve been streaming the same channel (CNN HD listed as AVC HD) via the Channels app from the Apple TV4 and from my iPhone 7.
The Channels app on Apple TV4 was using Tuner 2, and the Channels app on iPhone 7 was using Tuner 1.
Video froze and stuttered EXACTLY the same way on both devices at the SAME TIME. Both devices did this in sync.
Does this help narrow down the problem?