I was getting the same problem until I turn the "Target Bitrate [Mbps]: all the way down and the "Maximum Bitrate [Mbps]: all the way up. I'm not sure why this did the trick for me, but it worked. Here is a screenshot of my export setting for the Bitrate. I hope this helps you. I've looked and looked and no one ever said any thing that I could find about the Bitrate. Please, if anyone could shed some light on the "Bitrate Setting" it would be great.