99 jeep wrangler 4.0 auto.96000 miles. po138 code. replaced sensor,cleared code, have no pending codes. still stumbles. its worse when cold, have to put in netural rev then gos away. ounce it warms up it gets a little better, but gas mileage still sucks.put in new plugs,muffler, no vacume leaks, pcv is good. the C and the O symbol on the scan tool flashes on and off. any ideas?