Could be your air mass meter. I guess if you removed the wire that connects to it (check your manual first - I no mechanic) and then the symptoms stay the same (i.e. get no worse) then you could be pretty sure it isn't working.
If not air mass meter I should get the dealership to put a computer on it as there are lots of less common things (e..g exhaust gas recirc etc) that it could be.
If not air mass meter I should get the dealership to put a computer on it as there are lots of less common things (e..g exhaust gas recirc etc) that it could be.