First off I want to say good job on paying enough attention to notice the flutter soon enough to save the plane.
I don't think it's that your engine is to big as much as the plane is going to fast, I'm running a Saito 100 on mine and I've never had a flutter. I would say that even at only 60% full throttle you shouldn't be going that fast unless your plane is going up. Also what size and pitch prop are you running? I don't know much about prop size for 2 stroke engines but for this plane you should get the biggest diameter and lowest pitch prop your engine will turn.