If you are using a Pertronix ignition system I would advise to but the correct coil from them. You will also want to make sure that you have a full 12 volts going to the positive side of the coil. Also with the Pertronix ignition you must use suppressor core spark plug wires and not solid wire core wires. If it were me I would do a compression test to find out how things are mechanically with the motor. If you have more than a 10% variance between any of the cylinders that will indicate if you need to be doing some more diagnostics. I would also recommend checking the output pressure of your fuel pump.
