Check the reference sensor. This sensor is what tells the motronic
computer the engine is turning. It also affects timing and is what
actually allows the computer to properly time the spark. If it has a
broken wire, which I've had a few experiences with now, it can give
eratic spark or no spark depending on the wire touching ends or not.
Once you rule this little sucker out, check the cap and rotor. You can
check the sensor with an ohms meter. Just look for continuity through
the sensor and move the wiring that goes into it around. If there's a
borken connection, you'll find it. I've had two where the wires have
broken off inside the housing of the sensor.