fwiw, I actually don't think your stab is too tall. Mine are huge, and they do flex a bit, but they don't cause that kind of rocking. If you're concerned, put a single bamboo skewer down the flutes. I assume you ran the flutes more or less veritcally? Instead, the rocking is more likely just caused by the near-stall condition and the turbulance. Making the stab taller might actually fix it by getting more stab area up in to clean air. You might also try a sub-fin.

I agree with playing with the CG, btw.