If the autobox is quite old, and has done many miles, a complete change of fluid will make a difference. However, draining the autobox will not drain the torque convertor so the refill will be a mix of old and new oil but, this will be better than the box running on very old fluid. Also, check the fluid level with the engine running. There should be a "cold" and "hot" marking on the gearbox dipstick. There is an internal pump in the autobox, and it's suction side will have a very fine mesh filter. Take the box sump pan off, give it a good clean, together with the pump filter, also, remove any debris (swarf) which might be adhering to the magnetic blocks in the sump. If this does not cure the problem, it's time to take the car to an autobox specialist.
