Comment # 39 on bug 1028575 from Mircea Kitsune

(In reply to Ismail Donmez from comment #38)
> I can't see if in this bug report but did you try to set kwin to use XRender
> instead of OpenGL?

If you're referring to desktop compositing, I tried disabling it altogether.
This might make the problem slightly rarer, but it is not the root of the
cause: The freezes can still occur even without effects enabled at all.

The trigger always seems to be selecting another window or popping up a new
panel: If I put the mouse pointer near a panel to make it appear, or alt-tab
switch to another window, or switch to a different desktop... that's when
there's a chance that it happens. It never happens when I do something in any
application, only when I do something that changes the desktop.