I had the same problem with my 525i M50 engine. It turned out to be the spark plugs. I was told to only use the specified plugs per the maintenance manual and that did the trick. I spent $40 US on platinum 4's and it made the car hesitate around the same RP you mentioned. The Bosch Super did the same thing. Do you have the correct plugs in? You'd be surprised how much the plugs affect the knock sensors with electrical noise feedback.