Comment 9 for bug 776654

this effects me also, and its getting rather annoying.
totally random times.

Is there any kind of fix for this yet?

@ Eugene Swan~ You say "Try to use sessions without compiz ("no effects") or simply uninstall compiz.
In my case this solved all stability issues."

That is wrong.
We are talking about 11.04 here.. with Unity..
which depends on compiz.
There is also no longer a "visual effects" tab in appearance window.
Therefore, no disabling them.