I have a W124/M106 engine. Similar problems. Done so far:
New OverVoltage relay, new fuel line check valves, new fuel filter, EHA replaced. (Mechanic said the fuel pumps were OK.)
Tune up. Injector cleaner (helped somewhat.) Good gas (though cannot get the grade the book recommends..... As only 89 or 93 octane available, and book calls for 91 octane.)

Cold start: Cranks at the touch of key, no pedal. Then RPM dips for a few seconds, then OK for long time.
Park in hot sun: Cranks at normal speed. Fire on plugs. Not starting. Wait & wait. Still same. (If I wait 20 minutes, then it will start.)
But.... open the valve cover-to-air cleaner tube, spray a few seconds some ether/quick-start, and presto! Starts right up !!!!

Have suspected the "7th injector" but not sure. This injector has voltage at start-up.

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BnObob (for the fallen flag railroad.)
Artic "Snow" White 1989 300E, 124.030
w/ M103 engine, and 106K orig miles.