I have seen this before, My case was due to audio and video settings… I have found strange issues with the cube not liking 4k video and/or audio other than g.711mu… I tried so hard to get all of my cameras to show on firetv and echo’s using my stream #1 4k output… I ended up setting up stream #2 on all of my cameras and set them to 1920x1080 HD ouput with g.711mu on the audio out of the camera, then @noaudio and @proxy using the gateway. My NVR still uses the 4k stream #1. At this resolution the cameras are all stable on firetv and echo’s (including the cube), i can start the video and leave it for a long time, no buffering or cut-out issues.
example of how one of my cameras are setup.
Screenshot from 2020-02-09 13-41-09.png