I've got a new Amazon Fire TV with 4K Ultra HD box and over the last week or so some videos have started intermittently switching between a smooth display and a blocky display during playback.
I've only noticed it on old 720*576 (4:3) TV shows so for example I'll be watching the attached Doctor Who and one minute the picture is good (on my 138" screen) and the next minute it's blocky/pixellated for several seconds (you can see this clearly around the soldier's eye and in the background.)
Does anyone have any ideas where to start troubleshooting this?
The Fire TV box is hard-wired, the video is on my local server and the video plays fine via both the Plex web client and a standalone media player.