Actually I'd say you've got too much downthrust. The engine is pulling the nose down and when released by idling it comes up.

But you've also probably got too much decalage as shown by needing down trim to balance the engine's power.

In fact I'd say the best action at this point is to give the whole model a very serious alignment check for all the incidence angles and thrust line angles. It sure sounds like something is out of place. 1/2 degrees count for a lot on models of this type so be very fussy and exact in your measuring methods.