I have a 1984 300SD that did that for several years. After a long run with a hot engine, the entire car shakes like crazy. Recently through MB archives on the internet, I dicovered that the fuel rack damper screw was the most likely cause. This damper screw is on the bottom of the back plate of the fuel pump. I've read that it is very common for the older style rack damper screws to be fatigued. A new screw only cost me $8 here in LA, Ca. I removed the old screw, put in the new one(not tight). I ran the car until the shakes came and slowly turned the damper screw in until the shaking stopped. Then tighten down the lock nut. At $8 it seems like a easy thing to try. Now matter how hot or hard I run my car, no shakes, no starting or idling problems(270,000miles)... Simply a great fix. Good luck with your diagnosis!