Remeber, it is a trainer and once you have it flying level at about 1/3 throttle it is suppose to climb when you add more power. That is one of the features that make a trainer a trainer. You can dial out some of this characteristic by changing the engine thrust angle or add a shim to the rear edge of the wing to change its angle.
Be carefull though, if you over do it that will make it harder to get the plane off the ground.