Torque, load, etc etc etc all play into what gear your pcm selects. I'm merely suggesting that before you swap a trans to find out it's not your issue. That you might want to rule out that maf sensor first.

Let's pretend in Dec I swapped a perfectly fine 186K mile trans, because of a coil pack. I couldn't see any issues and changed the trans based on mileage etc. New trans did the same stuff, I shook my head and started shotgunning parts until something worked.