I was driving quite casually for a while in my W221, and then when I finally got some space I decided to accelerate at around the 40 mark and noticed no power was felt, the rev meter almost stayed at 2 revs if not lower and the car slowly accelerated. I even put my foot down to the floor and this made no difference. Tried it in manual, same thing. Parked up and switched the engine off, after a while I started again and the problem had disappeared. Accelerating as normal. Any idea what it could be? I have a health check booked in next week and will see to it then, at the mo its driving fine.