How many pickup coils do you have in the distributor, one or two? You can tell by the number of pairs of wires plugged in. If there's two, there is a relay that switches them in and out. It likely has a burned contact or corroded terminals. One of the pickup coils could be open intermittently too. They often fail when warm and work again when they cool down. One coil is only used during cranking to provide retarded pulses for easier starting. When you release the ignition switch, the relay kicks in the "run" pickup coil. To verify that, just switch the two plugs between the two coils. If it starts okay during cranking but stalls when you release the ignition switch, you have a defective pickup coil.

The air gap for those pickups is critical too. As I recall it should be .018" and must be checked with a brass feeler gauge. If the gap is just a little too big, the ignition module may not be triggered by the pulses due to the lower system voltage during cranking. The clue is it WILL start when warm if you connect a battery charger.