Oh no - don't be confused.

Its ok if you're not losing power when cruising.

Just focus on the valve control and see how it goes. We'd take it 1 step at a time.

Clean the valve as suggested by YMSIN but no improvement. Will send Benz in for diagnostic on the scope soon. Thanks to all who reply to this post.

Sorry lancel.

I was gonna say that the valve cleaning did some good (or so it seems to my ole 300SE). The idle has steadied where it should. Perhaps the valves were dirty after all. Unless of course the idle decides to play up soon.

Let us know how your diagnose went. Would be keen to know what the experts have got to say.

Update:
while cleaning the idle valve, I notice that the adjustment knob for the aux air device was broken. I manage to find a replacement for the knob from a junk yard. Some how this has improve my 190E rough idling, though not completely cured as I still experience slight mummurs. Please explain how an aux air device (which is connected to the idle control valve) function and also how it work.

I guess its a regulator to stablise the required ait-intake. It may be the feeder into the idle valve.