Here's the deal I have a 69 Chevelle with MSD Pro-Billet ready to run Distributor, 6a Box, Blaster 2 coil. Every once in a while the car will crank and run, shut it down and try to crank it again then its starts and almost imediately shuts off then try it again and just turns over. We have tested the box and coil and get spark also replaced the magnetic pickup. ONe concern is there are two wires that are used for the switched 12 volt, one is the resistor wire and the other is a brown wire, I understand the resistor wire does not provide the full 12v to the box but the car has ran like this for 2 years without any problems. Also has fairly new ignition switch and lock cylinder.

Turned out to be the Battery...never would've thought

Your alternator must not be charging if the battery died! Once started, the alt should be able to keep the car running.