The car has the standard carb. It has a Lumention ignition add-on which replaces the points.

What have I done to troubleshoot...
Ohmed out the ballast resistors.
Checked voltage at each wire according to the wiring diagram in the MBZ M110 engine book.
Renewed ground connections at battery, under coil mounting clamp, and engine ground straps.
Removed and reconnected all connections at alternator and sprayed with De-Oxit.
Tried two other coils.
Renewed the connections at the test block right by the coil.
Ohmed out the wire from the starter to the ballast resistor.
Jumpered the wires to the neutral safety switch together.
Pulled the plug on the back of the ingition switch and hot-wired it.
Have connected a timing light to the coil wire. No flash = no spark, right?

On the rare occasion when it will start, it seems to run perfectly, smooth idle, smooth rpm ramp up when gassed. When it dies, no sputter, it just quits, as if I turned the key off.

Neither the engine manual nor the CD have anything about the starter, solenoid, and the wiring related to either which I'm curious about because of the voltage spike that I've read about that should be present while cranking. The only time I've seen more than 4.5 volts at the coil + terminal is when the car starts. I've seen anywhere from 6.5 to 11-12 volts very briefly there, again only when the car starts. The last time for this was a month ago.