I think you’re right .
The original was I believe 1080 x1920 at 25p
However every time I converted from H264 to Pro Res I got an Upper Field frame result and my sequence settings in my FCP timeline were None in field dominance , so I think this is why I’ve had the problem.
I converted the footage to XDCAM and this works without any of this terrible interference , although the definition seems lower than Pro Res 422 HQ.
I also tried dropping the Pro Res footage in an Upper Field setting timeline and it seems to work OK. but all my other material in the film is different so I can’t proceed .
DJI just told me that the Phantom 3 Standard is not compatible with Pro Res . I might just carry on with XDCam .
Derek