I agree. The incidence must be off. Again being a shoulder wing I would add shim stock to the trailing edge until it required up trim to fly level then back down from there. One other thing to try. When you drop the throttle to idle does the model drop its nose? Do you have a incidence gauge? If so set the wing at zero and lay a level across the mounting lug of the engine and make sure it is level too. If it is pointed up there may lie your problem.
Bob Harris