Mercedes: 1979 450sl, fuel contamination, moisture absorption

Question
The car starts cold perfectly with a high idle, and will run fine for about 20 minutes or so. After that, during acceleration, it has a definate miss and major power loss. The longer it runs, the worse it seems. Supposedly there is no catalytic converter on this particular car. All the basic tune-up work has been done, including all fuel filters etc. Fuel system treatments have shown little or no improvement. Could this be cause by a leaky or restricted fuel injector?

Answer
I would look to make sure the Catalytic converter has been removed. It it is contaminated or partially restricted, it can cause the symptoms you describe.
You also may have fuel contamination with water. Make sure the gas cap is new. With the new ethanol laces fuel there is more opportunity for moisture absorption and water to collect.
I would add a bottle of black bottle tecktron injector cleaner to a tank of premium fuel, preferably without ethanol.
I trust that new plugs were installed as part of the tune-up.
Let me know what you discover.
Robert