I don't use Movian, though I may have to give it a go.
Superceleron specifically recommends a heatsink when using his ROMs on the Vim. I don't know if that contributes to your playback issues or not. For testing, you could use a fan on the Vim to rule out a heat issue.
Curious, do you also have these issues using the included FTMC?
Have you played with Auto-refresh setting and/or HDMI Self-Adaption setting?
Hope you get it worked out.