Between 1000 and 2000 revs, revs rise by about 100 revs every few seconds. Over 2000 revs this stops happening or is unnoticeable. Acceleration is almost always lumpy, a bit like the car is being starved of fuel. Two garages, including main dealer, have tried to fix this, but not been able to find the cause. Any ideas? A4 3.0 petrol Cabriolet Automatic.