I'm having the same problem and in my case it's somehow linked to running kopete. Whenever the desktop freezes, kopete also freezes. Killing kopete unfreezes the desktop again.
-- kdesktop freezes often during RMB actions (dapper) https://launchpad.net/bugs/43065 -- kubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/kubuntu-bugs
