I bought this car from a guy not taking very good care of it, among the major problems with the car the engine was stalling. when it was cold it would stall a lot, in warmer weather only a little. i pulled all the sparkplugs and out of the eight of them the left four were good the right for had the two towards the front were clean next one down had a little oil and the left rear plug was cased in oil, top to bottom. the car runs great after it warms up though, and i know that an o2 sensor is broken. the car idles at about 500-600 rpms but if it drops below 300 it stalls out. what could be the problem?