I have just had this exact same problem at the same revs and it was the air flow meter. After replacing the meter the car was instantly restored to full power and it came as quite a suprise to feel the difference. I think it was probably gradually dying for a while and I did not notice until it became quite bad.
I was a bit cheeky and asked my dealer for a discount which he gave me. 10% knocked off so it came to £153.