I reckon it's the MAC QuickTime Viewer that's the problem - and can't handle the 60fps ..... can handle 30fps fine.
Prior to attempting to upload a 5 min segment ... I took note of the time into the video that the stuttering occured.
First time it was - 55 secs, 1.52, 2.13 etc
Second time it was - 35 secs, 1.05, 1.35, 1.55
So different segments were stuttering the second time I checked ... so it must be the viewer.
If it was on the SD card ... it would be the same both times.
Rather than go out and buy a New Retina 4k MAC ... I dropped it down to 30fps .... as much as I'd like the new MAC