what rom are you using? I think the only kernels that don't use the cache-bug-workaround are the "special kernel" and the one in watapon. Truning off the cache-bug-workaround makes a big difference.
Also, do you have libmad installed and working? Video will get choppy if the Z uses floating point instead of integer operations to decode mp3--ad mp3lib(?) or whatever the default is uses floating point.
ssh in and run the (55) program.. read the output to see what lib your z is using to decode the audio.. you might get rid of a lot of choppiness.
While I'm at it the free kino2 (latest release) with the new mpler w100 (though you'd use the framebuffer on a 56) Is really really nice...and since that mplayer is statically lined, you get libmad doint the mp3 decoding off the bat, as well as ogg, ang ogm video..