I Have A 2005 Mini Cooper S With A Problem In The Automatic Transmission.

I have a 2005 Mini cooper S with automatic gearbox in Medellin Colombia. The car was purchased in the US. The car runns well the first 15 minutes I start driving the car in the morning. After around 15 minutes the gearbox starts to fail. When changing gears from 4th to 3rd and from 3rd to 2nd that change is not smooth, but rather hard and dry like changing a manual gearbox without using the cluth. In the authorize BMW dealer they say that they have already change the wireing, the electronic devise that controls the gearbox and that now the only thing left to change is the gearbox itself. Is this the case? I don´t think the gearbox is broken because it runs well for a moment.   What can you tell me about? Who Can I contact?