I think I have figure out why the plane hunts.
The linkages are ball links and ball swivel links. There is zero play or slop in the linkage. When I hold the bell crank stationary and try to wiggle the elevator, it does not move a bit. But here is the kicker. I can rock on end or the other of the bell crank up and down and the elevator moves a 1/4" or more. It is just a standard nylon bell crank with a nylon bushing, and can wobble a fair amount on the bushing.

I will make a new bell crank from some brass or thin steel stock and solder a bushing in the center and then thru bolt it. Whatever it takes to get rid of the rocking. I think the plane is actually nose heavy, but the elevator hunts up down and so the plane hunts up and down. I have never encountered this before, but I also have never had a totally zero slop linkage before.

So a new bell crank will be the first thing to try in taming this plane.

Ken