I have a 3.9L v6 2wd dakota. it has started to act up in the past few months. it idles rough when it is cold and has misfires when trying to drive cold. misfires happen at about 1000 rpm and the truck does not respond when pushing the gas, even flooring it. when coming to a complete stop the idle shakes the truck. engine light has come on and took it to a mechanic, but the light turned off. finally got the truck to the mechanic when the light was on and the diagnostics showed a misfire in cylinder 1 and o2 sensor going out. mechanic cleared the computer and hooked it to the truck but the truck light also turned off and the computer picked up no erroe codes. 10 minutes after driving it away and the truck misfired again at 1000 rpms, about 4 times. mechanic baffled as am I. please help