I have heard of this problem before and was told that inside the autobox is a mesh filter that tends to get blocked. This prevents the gears from engaging correctly. To cure this, drain the autobox, fill with a cleaning/flushing product then run the engine for 20 to 30 mins selecting all the gears. Drain and refill with the correct oil. Was told that it HAS to be the genuine Ford Auto oil.
Kevin, December 2005