I believe that JCL is on the correct path here. When I had a faulty wiring harness in the valve body that was causing shifting problems (not unlike a faulty MAF) the dealership per the TSB refilled the transmission with the fluid that came out of it. It was explained to me by the shop foreman that if all new fluid was introduced to the transmission it would shock it (the transmission that is) and could cause dislodging of sludge, debris, etc that would cause problems for the transmission.

If you do drop & drains at certain intervals, your likelihood of having issues decreases.

__________________
03' X5 4.4i Black Sapphire/Black
04' Land Rover Range Rover HSE
95' Jeep GC Ltd