What you have sounds like a stereotypical problem for that vehicle. I quote (without permission) from Frank Barrett's fine "Mercedes Benz -- Illustrated Buyer's Guide". You should pick up the book. Its ISBN 0-7603-0451-3 from MBI publishing.
"The 450SL had excellent performance, but early emission controls strangled the engine. Performance, drivability, and fuel-efficiency suffered. The 1972-1974 U.S. models could use leaded or unleaded gasoline, but 1975 and 1976 models had the most vapor lock problems because their catalytic converters were in the engine compartment, where airflow was restricted by the tight-fitting engine hood. For 1977, the catalytic converters were moved back beneath the floor. [...] To help prevent vapor lock, for 1980, the air conditioner was used to cool fuel being fed to the engine. Earlier cars can be fitted with a switch to run the electric fuel pump and circulate cooler fuel from the tank before starting. It helps to keep the tank full, if possible. If vapor lock occurs, try turning the ignition key off and on rapidly. This starts the fuel pump and may move cooler fuel into the injection system. Another way to do this is to disconnect the plug on the airflow sensor safety switch and let the fuel pump run with the key on to flush the system. If all else fails, pour cold water on the injection system."
I can say that the fix they did in 1980 must've helped, because my 1980 450SL has never acted up in this regard, even on the hottest summer days. Good luck.