I have an acquaintance with a 582 powered kitfox that had this same
problem...It turned out that the prop was very slightly changing pitch....I
remember it took a lot of time before this was found out. There was wear in
the hub area. It was fixed by adding a shim. It seems like (I'm trying to
remember here) the prop hub was slightly distorted where it clamped
together. What type of prop are on the planes? Are they using GSC props?

--
Yosimite Sam