+1 on a taper being a possible cause. I actually lost a prop that way, first year I had the boat it would clunk and slip, then come tight. I thought it was the tranny and was going to get at it at the end of the season and then one day as my wife backed the throttle after dropping me at the dock the boat seemed to be in neutral no matter where the throttle was. Turns out the lock nut had spun off and when the load was off the prop it just went to the bottom of the lake. I can't speak to who if any of the manufacturers actually lap the tapers but if they aren't lapped they can come loose. On my boat there's only one taper (Supra Rider) and it's at the prop end. If anyone has pulled the coupler on the keyed type, it should be replaced, they wear and will be loose if put back on. if it's tapered, it should be lapped.