Hmm, it sounds like you may just be CPU bound. Playing 1080p50 h264 without hardware acceleration on an older machine (I assume, since you’re on 10.6) may just be more than the machine can handle. I’d like you to try a transcode just to rule out codec incompatibility.
You can contact us directly at support@divergentmedia.com if you want to dig in further – it’s a bit easier to handle a thread like this via a ticket.
Hey all – Colin from Divergent Media here. We do have a clipwrap beta which adds support for the lpcm off the nx5u. If you want to give it a try, just shoot us an email at dmsupport@divergentmedia.com. Otherwise, look for a release before too long.