I got an S912 box (Mecool M8S Pro L), which had the video jerk slightly every time a subtitle appeared on the screen (embedded srt subs in an mkv container).
Aaahhh..... okay.
After reading several posts and going through a lot of discussions, I was getting the feeling that only Minix U9 should be bought if you go for S912, which amy not true at all but worked for me absolutely fine. The only complaint I have (if I'm really critical) with the power button, being on the same side (and too close) to the USB ports, which I hit unintentionally that ends up with a reboot every time I do that mistake. I'm sure you will be just fine if buying 'another' is an option for you.
-San