John, is the engine mounted upright or sidewinder? If up right, somehow block off the center hole of the saucer---tape etc. Be sure to do both sides. A question??? When flying inverted, does it dart toward the ground, or still towerd the bottom of the saucer? The engine could be blocking the air passing through the hole somehow in a single direction but not in the other. By temporarily blocking off the hole you can check this out.
DUCT TAPE!!! Another use!!!
Bigiron