My truck has a surging between 2100 and 2500 rpm when driving I can only tell it is surging when driving and it does not do it what i can tell when it is in park but does do it in all gears. It threw a code for the downstream o2 sensor running lean a while ago. Is this a bad o2 or would it be a vacuum leak causing it to run lean which is more likely or am I way off?