OK, I checked it: so far compositing has been turned off.
So I turned it on following this guide: http://ubuntu-tweak.com/2008/02/10/turn-on-the-compositing-feature-of-metacity.html and everything is still fine. Rebooted: still running without any problem.
So I turned compositing off again (and compiz on): still running.
So at least here it seems to be un-related to compositing.