I believe we have found the cause of this--items being rated need to be self-ending. And a recent bug was causing movie clips to NOT continue on to the next item when they were done. The consequence was that MediaLab would appear frozen, when in fact it was simply waiting for the item to end. Anyways, this issue with the video ending not triggering the next item appears to be resolved now and so you should be able to run your study now.

See also: http://www.empirisoft.com/support/showthread.php?t=4980
See current version 2008.1.33e or later: www.empirisoft.com/download.aspx