You are probably stalling out the wing, putting it past its "critical" incidence and creating too much disturbed air over the top of the airfoil. Check incedences, reduce throw, mix opposite rudder of the rotation in with elevator, play with you cg a little, and/or check both elevator halves to make sure they are in perfect alignment. Check these things one at a time. it could be as simple as Minnflyer's suggestion, I would check that first, but it sounds to me you are stalling the wing. At that point you have these forces working against you:

1-Engine torque; When the wing is stall there is no lift. The torque from your motor will spin your aircraft in the opposite direction you engine is throwing the prop. The only way to stop the effect of torque is to add opposite rudder.

2-Gravity; Your airplane is starting to fall out of the sky.

3-Forward inertia; Your aiplane is moving forward, but in a downward ark because of gravity.

Sometimes more throw will get you out of this situation, coupled with LOTS of LOTS of LOTS of POWERRR!!!

Agian, check lateral balance and elevator halves first.

My opinion.
Bipes