You need to let Vero output video in the resolution and refreshrate of the video stream. As a minimum:
- set your GUI resolution to 1920x1080 50hz
- set Settings->Player->Videos->Adjust display refresh rate to ‘On start/stop’ - You will still get 4k video playing as 4k.
That should fix things but you may have a better experience using the whitelist so that SDTV plays in its native resolution. For that have a look here.