Is your engine counterweighted? Is there any rpm limitations on your prop engine combination? What is the configuration of your air intake system and does the descent procedure change the relative air flow around the intake in an unusual manner? Are you on mags or Electronic Ignition? If mags are they in good condition ie no arching evidence inside the caps, plugs normally dry or wet, stuff like that? Unusual bends in fuel line going into the carburator? These are some ideas to explore. Bob Smiley N94RJ