try cleaning the inside of the throttle body, in particular around where the butterfly seats. If you look into a throttle body, you will notice a band that is machined much more acurately where the butterfly closes. Any amount of residue building up in that area will cause disturbed airflow at idle and cause rough running.
Proper way is by removing the throttle assembly from the car so you can get at the "engine" side of the butterfly, but you can do it in-situ. If you remove the elbow between the throttle body and the airflow meter and hold the throttle open, you can still clean through about 90% of it. Use a rag wrapped around finger and petrol, nothing more is needed, don't use anything metallic, you may scratch the machined face.
Not saying it will fix it, but I've cured quite a few BMW's and a 911 with just this.