Still sounds to me as though your problem is too much elevator travel. When you are doing a square corner or a tight loop you are pulling more elevator than a normal maneuver requires and the plane is snapping out. This type of plane needs very little elevator travel to fly pattern maneuvers. Try setting the low rate elevator for less throw and see if it helps. Stiffer rods won't help this condition. Your linkages are probably fine or you would most likely be experiencing flutter, which is a totally different problem.