I've been using Kubuntu for years and never encountered this problem. I am
not sure why you are having it. I don't think that it has anything to do
with Kubuntu itself. I would rather not speculate without knowing more. You
say nothing happened with the GUI. Does it just sit there and do nothing?
For how long? What processes are you running? Does this happen every time or
just sometimes? What other desktops and window managers are installed?