I experienced the problem just once and cannot reproduce it, anyway I updated my kernel to the following
Linux edubuntu 3.2.0-20-generic-pae #33-Ubuntu SMP Tue Mar 27 17:05:18 UTC 2012 i686 i686 i386 GNU/Linux
and, if it occurs again, I'll signal it.
I experienced the problem just once and cannot reproduce it, anyway I updated my kernel to the following
Linux edubuntu 3.2.0-20- generic- pae #33-Ubuntu SMP Tue Mar 27 17:05:18 UTC 2012 i686 i686 i386 GNU/Linux
and, if it occurs again, I'll signal it.