I installed Ubuntu 12.04 for a friend of mine and she often complains about freezes. I never saw these freezes myself, so I can only give the description she gave me:

• it happens when switching between multiple instances of an application via Unity’s Launcher
  
  
  
  
  • after the click at the Launcher, she sees a preview of all running instances; when selecting one, it zooms in, and then Ubuntu freezes
  
  


• it happens for Evince and LibreOffice (but it’s rare that she opens several instances of any other programs)


• if it freezes, she is still able to switch to the virtual terminal (Ctrl+Alt+F1)


• but: it can’t be reproduced; some days it doesn’t happen, some days it happens 3 times per hour

For a long time it happened with LibreOffice only, so I searched for possible solutions and disabled anti-aliasing; after this it never crashed again when switching LibreOffice instances, but it’s not a long time ago and so it might just be luck.

Any idea what could cause this resp. how to fix it?

Anything I could do to investigate it? The problem, as I said, is that it never happened when I was around; and we never managed to reproduce it.