my 96 disco diesel auto loses power at over 40mph ..... the engine kind of cuts in and out like it's running out of fuel .... the revs are there, you can rev the engine but it doesn't go any quicker, this only seems to happen above 40mph .... any ideas what the problem is ? ..... just hoping it;s not the turbo ? though im told that if the turbo goes you get a lot of black smoke out the pipe, which i am not getting .... thanks....