Guys, thanks!
Darren, this is exactly what I needed, now I can try if this option will have an effect: --enable-largefiles
Thom, I know we're not supposed to use Mplayer to play video files, and that support fro Mplayer is in early stages, but a mkv of 8 Gb in 1080p (the system and screen resolution is 720p, the file is 1080p) just won't play correctly with Xine (I'm on a Media Live with 169.12 Nvidia driver and 4800+ X2 CPU). I tried every suggestion I could find (enable useevents, enbale/disable sync to vblank etc). When I force it to play with Mplayer it works fine (except for the fact that I can't seek). If there is anything else worth trying please tell me.
I also have similar problems when playing back HD shows recorded by Myth, from the video section, which uses Xine, versus playing them from ithin MythTV.
EDIT: --enable-largefiles is actually not a command line option to run Mplayer, but a configure option when building it