75 280z Need advice on what this could be

I bought the car recently and replaced the coil, plugs, wires, battery terminals. The car is getting spark and fuel. If i hold the key on the start position(where the starter is turning the motor) it will rev up and seem to run. But whenever i release the key and let it go to the run position, the car shuts off. I have no idea what it can be, mabye the ignition or something electrical. Anyones advice will be greatley appreciated.