Lumpy saab 95 2006 idle?

we have a 2006 saab 95 aero with auto gearbox. the engine is a 2.3 litre with a stonking great big turbo on it. when at low revs, stationary or slowing down gently the car seems to throb and lurch almost, it doesn't move but it seems to shake and wobble. the revs pick up and drop and aren't steady. before we get it checked i would like to know what sort of problem this could be...