I had the GP Extra 300 .60 and thought of it as an excellent flier. It never tip stalled but only gentle and expected straight stalls, it never snaped, or showed any bad habits, in fact it never did anything I didn't ask from it. I really loved that plane and had the best times with it.

Have you check that your wings are not warped? Have you checked lateral balance? Check that the elevator halves are syncronized.

Good luck