Just a swing in the dark here...you obviously know way more about Auto trannies than I, or possibly anyone else on this board, does.
The only thing I can offer up, how are the engine and tranny mounts in the car? I know that with the manual trannies worn mounts can cause if to pop out of gear becasue of the excessive movement...maybe if the same thing is happeneing here it's causing excessive wear on the trans? Maybe momentarily going into neutral or 2nd?
Again, just a shot in the dark, but it's all i can offer up with my limited knowledge.