I would agree with eveyone else that has said CG. Flying straight and level with just enough speed to keep it straight and level, as you SLOWLY back off of the throttle, don't touch anything else and see if the nose goes up or if it is actually the tail dropping.

I did some experimenting with an OMP 80" profile. With a rearward CG, I could get it to act as you are saying yours has. When i slowly let off the throttle, I could see the tail drop.... At one point, The CG of my plane was at the aileron hinge line......boy was that a fun ride!!!