Ford Falcon: Clunky gearshift

I have had nothing but trouble with the 20 06 BF MKII Falcon XR6T I bought in 2009, and which has just clocked up 38,000 km. My current concern is with the six-speed auto, which really bangs into sixth gear at around 80 km/h when cruising and has now started to clunk when changing down into first coming to a stop. Ford had a ZF technician check it a few months ago, and I was told that the tyres needed changing, that they were somehow causing the sensors for the auto to get confused. Following their advice I had four brand new tyres fitted, but surprise, surprise, it didn't fix the problem. I took it straight back to Ford, but they couldn't find any fault when they checked it. This time when they contacted the ZF technician he suggested the problem could be caused cause by outside influences, like contaminated fluid, modified PCM, etc, and it was no warrantable. They did mention that they could see the line pressures where high on the 5th to 6th shift. It was at this stage that I sent a letter to Ford, asking what's going on, to which I received a reply saying "we are satisfied that your vehicle is operating to manufactured specification and is similar to other vehicles in the model line" So how can Ford release a car the bangs into gear?