I have a 2004 Chevrolet Silverado 2500 HD. The problem start with the shift from 2nd to 3rd. When you get on it hard it will not shift untill you let of the throttle. Shifts fine while driving normal. Now when you are doing like 55 and you step on it it will down shift and only do about 2300rpm and go and faster untill you let off the throttle and let it shift up. Any idea what is wrong?