'84 380SL Engine Surge

Once hot, engine surges between 400-900 rpm at a two-second period. When accelerating, valves make little noise and acceleration is sluggish. This never happens when cold. I checked for vacuum leaks, found none. From browsing related topics, I found that an Idle Speed Valve can be responcible or its regulator. I've heard MB dealers call it the Cold Start Valve/Ramp Valve/Throttle Valve. The dealer cost for the valve is $435 but when I ask for the voltage regulator noone knows what I am talking about?

Is there a VR for this valve & if yes, where is it located?

Thanks.