Playing MP4 or Mov files in preview window stalls the software

I have the dual window up, and then preview on right hand side. When previewing a MP4 or MOV file, the application stalls for about 30 seconds after the movie has played before it comes back to life. You are also unable to move away from the previewing screen while movie is being played.

I have the latest version of the software, on Windows 7 64 bit

Neither format is handled by Opus itself. They're played by the video components installed on your system. The FAQs have suggestions on fixing them if you're having issues.