My 1972 250C had the exact same issue, stranding my wife once and me on another occasion. In the process of restoring the car this winter, I found several problems that may have contributed:

1. The heat risers were stuck, which likely overheated the carbs and definately warped them.
2. All the wires to the starter were free to move about, and combined to their close proximity to the exhaust manifold, the insulation had melted. It's possible, I suppose, that these were hsorting out and causing intermittent failure.
3. Frayed wiring to the original Mercedes ignition box.
4. The ignition coil, rotor, cap, and wires were all replaced on principal. I installed a Petronix Ignitor kit as well. I replaced the carbs with a set of Webers. I also rebuild the exhaust manifold and heat-risers.