Unfortunately, I have had the same kind of results with Kx focusing in AF-C mode. I think you should use multipoint AF anyway if you are shooting planes in the sky.
Speculating, I might say that maybe the AF speed just cannot cope with the high speed bursts or you are getting blur from the mirror slap
so using lower speed bursts, you might have more keepers
Pentax is not for high speed AF jobs, that job is better delivered by other systems. I am curious about the speed and accuracy of SAFOX IX announced for K-r.