Video VideoPlayer output glitchy sometimes

I'm using the new Unity VideoPlayer component to render videos on top of ImageTargets with the Vuforia addon. Sometimes the videos have certain glitchy effects like in the screenshots below.

Using Unity 2017.1 and iOS.

I have not yet managed to track down the exact circumstances of when this happens.
Seems a little random and sometimes seems to get better after the video has looped a couple of times.