My '91 300SE eventually starts, 8 or 10 attempts, and I can hear the starter motor turning if I hold the key in "start" momentarily after the failed start. this is usually when cold but after the car is driven to town, 15-20 miles it will start 1st time til it gets cold again. Commutator or...... I have installed a new solenoid and did have the NSS bypassed, no help. What am I overlooking or don't know about??

Thanks everyone

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1991 300SE (my ride, 279,000 miles, looks almost new
1954 Cadillac (21 yo son's car, he bought when age 15)
1972 SeaBird 19 ft runabout (old but solid, slant six, Volvo sterndrive perfect condition, undergoing complete overhaul and refit)
1998 Toyota Rav4 (my sons daily driver when he is in the Continental US, PROUDLY serving in US Navy)