RE: Diving on downwind?
Well the obvious answer is that it has a high wing loading. The reason that it's diving is that it needs more air to stay aloft. If it's falling on downwind apply more power. If you can't get enough power then your engine is not powerful enough for the plane.
What is the wing area and weight? Actual weight, not what it says on the box. Also check the RPM of your prop. If it's just happening then maybe your engine is losing power, even if it sounds like it has power.