[darther]

The U9 on the X8 HDMI / TV Input had the same jerky playback issue.
Installed XBMC for Minix 16.3 and the problem resolved, as i suspected.
So it's the decoders. 
The only issue is not sure how stable XBMC is and I'm yet to test 4K playback.

Edit: SPMC has the same jerky playback and XBMC refuses to play 4K in anything other than 1080p (I changed my Android Display settings before trying).

One other thing with Kodi, there are moments during the playback when the screen goes black for 3 seconds or so.
This doesn't happen in XBMC.

So it looks like i'll have to jump between Kodi and XBMC depending on what I want to watch.
Not exactly optimal.

Edit 2: XBMC 16.3 is not stable at all. Crashes frequently.