So yesterday I removed the engine with the gearbox. I have brought it to a specialsit who will rebuild the gearbox. This way i will know that the box will be as new with no hidden problems. Unfortunately it is really expensive...... but at least the problem will be solved and the box will be as new! Used boxes with unknown history can give problems and then I will have to get the motor and transmission box out again. In this way it is right in one go.