Adding a sub-fin (ventral fin) will definately help. Add about 50% of the area of the existing fin and rudder. Also check and make sure there is no binding in your ailerons controls.

With that much dihedral, you have a lot of level flight stabiltity that your ailerons have to overcome to make it bank. It's a bit of work, but taking all of the dihedral out of the wing will make it handle MUCH better in the turns.

Jim