"Seems to me that your transmitter is in idle up mode. That is the throttle curve is like a V or U, or even flat line at 100%. It is for 3D and allows you to fly upside down where the blade pitch changes, but the throttle remains high on throttle stick movement up or down." 
If I remember the DX6I correctly it too has a flight mode switch at the top left. Perhaps the previous owner had the full throttle setting at selection 1 or 2 Flip it down to normal and see if that clears the issue. </p>