The engine in my 250 would often show heat soak in warm weather -- enough to make a mechanic who specializes in vintage Mercedes comment on it. He suggested installing an electric auxiliary fan like the ones available from Summit Racing or JC Whitney and setting it up so it can run after shutdown and cool the engine bay. I never did it, but I probably would have if I lived in Arizona.

As for RPM, I've driven my car regularly at a steady 75-80 with the engine singing away. Enjoy the chat! And I think your transmision is in top gear. (Doesn't it start out in 2nd?)