#sport cub s

Hi, I am finally getting into constructing my own RC aircraft. I had a look around and I stumbled across a few people that converted Guillow's balsa aircraft to RC. It looked interesting so I researched a bit more and decided I wanted to convert a 500 series Spitfire. I am currently collecting...

my Sport Cub S is going down at a fairly steep angle at 50% throttle on Expert mode. It seems balanced to me and I have move the battery around to no effect. The elevator looks like it is at the right level. How do I best trouble shoot this?