Co2 sensor could be bad.. Like Kenny said make and model makes a world of difference! I had a 2000 Mustang I did all the things you stated and it still gave problems.. pulled one of the co2 sensors and it was basically melted! work checking...
I would check the timing.
Providing the year and make of the car would also help in getting a better answer/opinion.
a vacuum leak is likely. a timed leak.....
