Hi John,
I can't offer much help but noticed you've recieved 0 replies. I just purchased Combustion3 for our studio and haven't tried exporting in those formats yet.
I used to use Combustion 1 at a previous work-place and we always set our workspace to "Custom" where you can set your own framerate, pixelAspect, resolution etc. Try using this setting and see if you still are having problems.