My trusty Dawes Giro 300 has carried me to and from work for the last three years, I recently treated it [and me!]to a new transmission, front and rear. A problem has arisen in that when when I am on the big front chainring and the 2nd and 3rd largest sprocket it automatically changes down when I go half a turn at the traffic lights etc to pick up the pedal, it never did this before. There is no problem going forwards. I know that big chainring to big sprocket isn't the done thing and the Giro hasn't the greatset chainline in the world, but it is a pain to go back to the small ring at every traffic light in a predominantly flat area. Any body any thoughts please?