[xubuntu] Frequent black screen on MacBook with XUbuntu

Hi,

My MacBook frequently blocks on a black screen when I'm using it.
Then, I have no other choice to switch it off using the power button (which is not so good for the hardware!).

I couldn't find any command to restart the X server for XUbuntu, so I couldn't try this.
Note that, if the computer "blocks" while I'm listening to music, the music is still going on, and some keyboard commands still work (such as keyboard brightness control, etc.), which let me think this may be a graphic driver / config problem.
(But CTR+Q and similar don't work.)

Furthermore, the problem occurs in particular when many things happen on the screen (e.g., a shell program is looping in a terminal, and printing a lot of information very fast).

The system has been working fine for several months, and the problem is recent, and more and more frequent; it's almost impossible to use the computer more than 15mn, which becomes pretty annoying.

Hardware
MacBook Pro 6.2 Mac OS X 10.6.7
2.66GHz Intel Core I7
4GB 1066MHz DDR3 SDRM - 2x2GB

OS
XUbuntu 12.04 64 bits
(with Mac OS, just in case…)


Would you have any hints?

Thanks!