I'm a happy(ish) owner of a number of Mercedes E class estates for over 20 years, and am now experiencing a new problem with my 2004 320CDI Auto estate. When starting in the morning and pulling away, power is sluggish; the auto gearbox changes down far too soon, and within a few yards I am in top with barely enough power to pull up the gentle incline from my house. Occasionally this will occur during a long run on motorways at a constant speed. If I stop the car; turn engine off and then re-start, all is normal again, and full power/performance is achieved again. Any ideas what this might be? I am not mechanically-minded, but very curious as to what the cause of this could be?