Been there, got the tee-shirt. Change the crankshaft position sensor. On my '86 420 SEL, it was a green cable from the ignition control box, drivers' side fender well, to the bell housing.
When it gets hot it will quit; after it cools down some it will start back working again (usually).

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Providing a home for these cars:
1951 Buick Special De Luxe
1977 Rolls-Royce Silver Wraith II
1970 Rolls-Royce Silver Shadow
1986 Mercedes-Benz 420 SEL
2005 Mercedes-Benz E 320 CDI
2012 GMC Sierra 1500 4X4
1927 Pierce-Arrow Series 80
1931 Pierce-Arrow Series 43
1926 Ford Model T coupe