Have 1975 450 SL that suffered the same problem.
I assume you have found the idle adjustment screw and it is already turned in all the way.
If so there are two elements that can effect idle speed that you need to check.
The experts assured me that my problme was caused by an electricly controlled fuel valve located at the idel control assembly. I changed it it was not the problem. If you change yours, let me advise you that there will be fuel under pressure up stream from it. So do it with a cold engine.
My problem turned out to be the thermostatically controlled slide valve also located in the idel cotrol block assembly. It was carboned up and jambed. This explains why your idel speed drops a bit when the engine is hot.

Good luck