Vintage Cars: 1971 Mercedes 300SEL 6.3, smell gas, gas cap

Question
Hi, my father has the 71 Mercedes 300SEL 6.3.  The car will crank up perfectly and run for 20 minutes, especially in idle mode, then around 20 minutes into running, the car cuts off.  After about 5 minutes, the car will crank back up and run the same for about another 20 minutes etc.  What could be the problem?

Answer
strarved for fuel
Too much fuel
altenator

DO you smell Gas?, DOes it matter if the lights are on, does it shorten run time?
I assume fuel is fresh and filters are clean

Try loosening gas cap and see what happens

A little more info would help me help you

Sudden onset?
How many miles?
Driven daily?