Diagnostic (specific enough to be worth a grain of salt) didn't really come into play on these cars until the early 90's (from what I understand). I would do a THOROUGH inspection of you ignition system, especially you cap and rotor. Pull it all aprat, and replace what is nessecary, or just replace it all. You don't want to have to replace the ignition module (=$$$). I had the symptoms you describe earlier in my cars ownership, and once I pulled everything aprat, the rotor button was broken of inside the cap. Somehow it would star cold, run fine, but if I shut if off (taking it to a store or something) and tried to restart it, it would just crank. After seeing that, I was baffled that it would even start at all. Good luck, and post research here, so that we can better help.

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1989 300E 144K