Hi, Im no expert but a car of this caliber should not be causing this kind of a problem.
It appears that the default idle speed is too low (500 rpm), could be a faulty sensor. It could be the default idle speed needs to be re-calibrated. As you mentioned that when it is idling a bit higher it is fine. Either way, I dont think it will rectify itself and if it does, it will come back as it ages. As I said Im no expert but, you should be firm in having it rectified. As a car of this caliber, it is not good enough and it is not normal. good luck :)