Are you rendering to a high frame rate and high quality?
There is a way we could definitely replicate the issue here. If it is not too much trouble,
You can archive the mix and send me the .tarc through dropsend. (http://www.dropsend.com is a great free way of sending files. You can send it to )
Archiving the mix allows me to open the mix exactly how you created it and with all the files used to create it.
Please see the wiki link for a more information on how to do this with a video included.
I can then open the .tarc file and hopefully recreate the issue here, get the dev team to have a look at it in morning and hopefully get a solution for you.
Please do not hesitate to contact me if you have any questions on this.