could you run the following command and copy paste what it gives you?
cat /var/log/Xorg.0.log | grep EE
FYI it echoes the content of the X log file and grep will make it so only the lines containing EE (meaning that there is an error) will be displayed
btw, how can you run mplayer if your system is frozen?
I don't really know about your problem with mplayer in fullscreen sorry. I know I had that problem once on a laptop so it seems pretty common and you might be able to find info on mplayer's forum.