I see. That’s odd. I’m not at home right now so I can’t confirm the framerate of the BD but I’m sure that (as with the vast majority of BDs) it will be either 24fps or 23.976fps. Perhaps, if it’s the latter, that maybe why you’re getting some choppiness encoding at 24fps?
There is certainly a problem somewhere as having your project framerate set to the same framerate as the source should not cause choppiness. I wish I understood FCP better to be able to identify where the problem might be. I’ll check the BD framerate tomorrow and report back.