I recently had an issue with my Jaguar X-Type in that a pipe from the turbo had cracked and the car belched black smoke out of the exhaust. That was fixed easily by my local mechanic, but the fuel economy went up by around 10mpg. Since them another problem has become apparent.

The car sets off fine, first and second gears are OK but in higher gears I notice that the revs rise rapidly then the car picks up and the revs fall off. If I take my foot off the throttle then it still picks up for a moment before engine braking as normal.

Is this just the turbo finally working and they all do this or is the clutch going?