Comment 4 for bug 787349

I've found a potential fix:

-Install Compiz Settings Manager: sudo apt-get install compizconfig-settings-manager

-Go to OpenGL plug-in & uncheck the Sync to VBlank option.

I am trying a the moment wether this may work or not :) but it seems that it has worked for others. I forgot to mention in my previous comment that i was having this problem in Gnome desktop not Unity. For more about this fix check these links:

http://ubuntuforums.org/showpost.php?p=10798466&postcount=50

http://www.preshweb.co.uk/2011/05/ubuntu-11-04-x-freezing-after-screensaver-active-laptop-lid-closed/