Some time ago I bought a "vigilante" frame from www.468.com
, and after rebuilding this unflyable quad several times, the only variable left that I had not changed out was the frame itself !!. Seems the problem was, that even though I had balanced the propellers and motors vibration was badly messing with the flight controller. The solution was to cut some aluminium strips to act as doublers/laminate on the original arms. They bolt to the center frame with the arm mount and extend out to bolt to the motor with the original arm. It is now one of my best flying frames ... amazing change. This mod should also work for the Q450 frame in reducing, or at least changing, the resonant frequency of the arms of the frame. Flashed speed controllers will also generally allow you to use higher gain settings.