If it fires when cranking and then shuts off when you release the key, it's probably a problem with the coil primary circuit. The ballast resistor is bypassed when starting and it's in the circuit when running. Try cranking the engine with a remote starter switch (or by jumpering the starter solenoid at the relay) and see if you have spark with the key in the "run" position. If not, the ballast resistor circuit is open. If you've been cranking it for a while, the plugs might be wet. You might want to change them out, too.