This exact same thing is happening to me. Very frustrating. I am running v2008.1.33, and am using a *very* simple questionnaire.
Whenever I get to the on-line rating item, the questionnaire ends, *then* the on-line rating slider appears. The slider works if you use the arrow keys, but nothing else does and MediaLab is otherwise unresponsive. And, I can't force-quit MediaLab through the Task Manager.
What do I do? I would LOVE to be able to use the on-line rating feature.