Occasional Engine Stall with Transmission in D For W124 230E
 

I've been experiencing very occasional engine stalls while the transmission is in D. It will be more prone to stalling especially when there is load w/o depressing the accelerator, eg. turning the steering while the vehicle is stationary. This happens when idle surges, sometimes dropping down very low w/o recovering. My idle speed is fine when the transmission is in P or N. Idling seems fine on other occasions. Any idea whats wrong?