that just happens sometimes in fcp. ive had that problem like starting and stopping and what not but it shouldn't render like that. its only temporary. should check just in case..
but if you have a gopro3 why are you shooting in 720p 60fps instead of 1080p 60fps? unless you did and just put it into a 720p timeline to be able to upscale a little and shift around the image then that would make sense