Hold the phone, CG has nothing to do with the high speed stall that you are describing.

It is caused when the airfoil passes its critical angle of attack. In other words, when the wing is headed into the relitive wind at too great of an angle, it will stall, no matter what.

Moving the CG will effect your elevators ablility to force the wing past its critical angle.

I had a Tecate biplane. It was prone to doing the same as your plane is. My guess at the time was that the extreme wing taper was causing the tips to stall early, hence the relitivly low speed of the "high speed" stall.

In the end, there is not much you can do but change the airfoil, fly the manuver faster, or use less elevator. Darn physics is so legalistic