at the cli, if you enter "mplayer -vo help" it will list all the v
utput drivers available.
then you can try different ones using -vo when running mplayer.
for example, I use the gl2 driver, like so: mplayer -vo gl2
take a look at the list of drivers and try some that look compatible with your setup. some of them will work, many will not. just for giggles, if it's in the list, try "-vo aa" or "-vo caca" (note that you won't want to use these as defualt though!