Distributor caps can have hairline cracks that will short across to another terminal or to ground. I would remove the cap first and with a knife scrap the inside of each electrode and remove the build ups, also the end of the rotor. Put it back on and see if it helps. You might also have a dead spark plug wire. Remove them one at a time and ground it out on the exhaust and see if you get a spark. If all checks out, wait until dark and with no light under the hood run the engine and check to see if there are any blue sparks going around the cap or from wire to wire, if so replace. This should help spending money on the wrong parts. Good luck.
