My friend has a '97 S320 with 118K miles on it. The car is in immaculate condition and has been well taken care of. Several times in the past week he has started the car and driven it 5 to 15 miles, parked and gone into a store. Upon returning to the car the engine won't start. The starter won't turn over but all the accessories work such as radio, AC blower, etc. After the car sits for 30 to 40 minutes it will start up and run fine. He has had the battery and starter checked and both are good. The car currently is at a reputable indie MB shop and has been for several days and naturally the car works fine for them so they can't make any diagnosis. Anybody out there got any ideas on this one? Could the ignition key/tumbler be a possible problem?

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1986 560 SEL (159K miles)