I have a 10 year old Smart Forfour that has done 63000 miles. When in automatic mode, in 6th gear, travelling at around 60mph, it regularly tries to change down to 5th, hesitates, but then continues in 6th. This is particularly bad in wet weather, when travelling at a similar speed, the automatic and unexpected changing down at speed is quite fierce. Shifting to manual mode will obviously stop this happening. Is there something that can be done? Will a gearbox oil change help, or is it the gearbox on it's way out and in need of an expensive repair?