The first thing you need to do is determine if it is a FUEL DELIVERY problem or an IGNITION problem. At the point where it will no longer start, you need to see if you still have spark at the plugs. My favorite way to test this is hook up my timing light and see if it flashes while cranking the engine. I realize not everyone has a timing light. If you are losing spark, I would say it may be the coil given what you already replaced. If you still have spark, get a guage to check the fuel pressure at the schrader valve.