Intermittent no start

When it refuses to start, it cranks over okay but will cough and spit and maybe try and start but doesn't. Replaced in the recent months, battery crank sensor, distributor pickup, distributor cap leads, coil ICM. New tank and pump also installed. When started it runs smooth but with occasional misfire like symptoms. The misfire can also be seen sometimes on the rev counter. I have a launch v431+ diagnostic computer which only showed up a o2 sensor fault (downstream). I have the sensor but not yet fitted. Any suggestions would be much appreciated.