I encounter the same problem on my Z.Every time I run XXMs with cover turning off to play mp3s,or watch video under X .It freeze wothout inform.
but now ,it seems a solution come.
I set "Suspend time" and "screen time" to ZERO via Light & power setting.
xmms can play mp3s good even turning off the machine.
I watch video exiting from X ,it plays pretty well.but sometimes ,i still freeze.I turn the battery switch from "operation mode" to "battery change mode" ,and then reverse,and then press "on/off" button.It come to life again.
i know the solution come with luck.i dont know what the nature.
Is there someone can tell what happens?