Similar problem here: I experimented with h.264-videos and found decent settings for the built-in media player. The same video is unwatchable in mplayer, video and sound stutter like hell and mplayer doesn't respond any more (or better: takes up to a minute to react to clicks/key presses). I found it gets better when I reduce the bitrate even further, but it still isn't really good and I reach a level where video quality becomes bad. So given the fact that the built-in player has a better performance and a nicer UI, why should I bother getting mplayer to work?