I was having a similar problem, this solved it.
I have a little one, so I have the screen set to lock after a couple of minutes. When I unlocked sometimes the display would be all black except for a superkaramba widget, and unresponsive to 'alt F2' and 'ctrl esc' and similar key commands ('ctrl alt backspace' and 'alt tab' still worked) I finally had it happen when I had the system activity monitor open, and noticed that plasma was in disk-sleep. Which led me to your post here.
Also seemed to help with the resume speed of unlocking the screen.