I have a problem with my 1998 C250 Turbodiesel Auto. I got the oil and filter changed and since then the shift between 3rd and 4th is a little rough and occasionally it drops to 1st on slow downhill sections. The tech has put the computer on it and it is not showing any error. Would the control unit in the gearbox be malfunctioning or is there some other reason. The car has done 102,000 miles and was working perfectly before.