1991 Mercedes Benz 300sl power problem

in the morning car stars rough for 5 seconds then smoothes out and will run most of the day perfect, after car drives for 40 min to an hour, at anytime the engine may start losing power and slow down to a crawl,(maybe 5 miles per hour) if I stop the car for 15 to 20 min and restart it, it will start running very rough with more power and will eventually (around 10 more minutes) start running smooth again. Every morning it repeats the same startup routine ....I was told it may be the crankshaft se-censor. I am confused because 80% of the time the car runs perfect (already replace, ignition module, fuel sensors, filters and pump, plugs, and wires

Thanks