Has the trans been "built" before? When you "shift" into the next gear, does it freewheel? If so to all of the above, you may want to drop the valve body to see if a governor plug has been installed. I used a "known" good case to use in a fully automatic situation. It acted as you are describing. It was a case that I formerly had used in a "manual" shift situation. The result was same as yours. Probably completely different situation, just throwing it out there.
