OK, I am making progress I think. I decided to pull the gearbox and send it in for an inspection and repair if necessary. While it was in work, there was some damage discovered although it was minor. The two Belville washers were flat and there was fretting on the step washers. There was some chatter marks on a spline for the drive shaft And all of these indicate movement in the gearbox. I had the gearbox rebuilt and I had a slipper clutch installed even though it’s for an 80 hp engine. This was a recommendation to also dampen vibration. Let’s see if it works. I have reinstalled the gearbox tonight and will try to get a flight test in the next week if the weather cooperates. Here in North Idaho , in the winter, it can be wild.