Disclaimer: My car is a 1988 560SL but I believe the fuel injection system is the same.

I had identical symptoms when I first aquired my car. I believe you have more than one fault. The high idle is probably a vacuum leak. If your car has an "ECONOMY" gauge (actually just a vacuum gauge) it must be pegged to the left when idling. If it is not you have a vacuum leak that will affect your idle among other things.

The driveability issue worsens when the engine warms thus indicating a temperature problem. In my case the culprit was the engine temperature sensor that inputs to the fuel injection computer. An easy DIY fix by replacing a reasonably priced and accessible component. I don't know where yours is located but you will find it screwed into the intake manifold somewhere. It should have two wires connected to the top.

Good luck and let us know what you find out regardless of the outcome.