Hi guys, wondering if someone can help me out here. I picked up a 91 500sl 3 weeks ago and love it. The only problem I have is an extended cranking time ocassionaly. Had a mechanic friend of mine look it over. It seems that I am missing spark out of both ignition coils. It happens very rarely. If I turn the key to off and try to restart it comes to life!! Any help would be greatly appreciated. BTW things checked that are fine:

Distributor caps,rotors, spark plugs, leads, fuel and fuel pressure