Mine did too at high speed. And in a knife edge.

It's actually doing something similar to a Dutch Roll, only without the roll. Both are tail wagging and stem from the same cause. Lateral stability.

Try adding a bit more vertical stabilizer. Developers do stuff like that all the time. Cut a piece of wood that adds from a half-inch to an inch worth of chord to the fin/rudder. You can masking tape it to increase the fin forward. It'll stay on for one test flight and show you if that's a sensible solution. If it works, just clean up the fin, glue on a clean piece of wood, and cover the sucker with matching film. Or don't bother to match the colors, and show the world how to improve the sucker.

BTW, if you do that, also be sure to test the airplane's spin recovery with the test piece in place. The vertical stab/rudder area also affects how easily an airplane goes into and stays in a spin. But don't worry too much about the Calmatos. Mine were easy out.