|This is almost always caused by a bad ignition switch or the wiring and connectors associated with it. Normally, when the engine is cranking, an extra contact in the starter solenoid bypasses the ignition switch, tach, and ballast resistor to provide electrical power for the ignition. Once the engine fires up and the driver releases the key so the ignition switch springs back to the RUN position, power comes from the switch and runs through the tach and ballast to the coil. But if the ignition switch or its circuit is defective, the engine quits.
There are a few things that are easily checked. Make sure that the connections to both sides of the ballast resistor are good and snug. Check the plug on the back of the tach. Unplug and examine both sides of all of the main wiring connectors under the dash, looking for broken wires and loose or broken pins.
If none of this proves fruitful, place the ignition switch in the RUN position and trace the wiring from switch to coil, checking at each connector to see where you lose power.