Have 1997 Mercury Mystique with 2.0 -4cyl, auto trans about 121k miles. Started ideling rough, wanted to die at stops. Check engine light came on after about a week. Had codes read, "P0171" came up. Website suggested cleaning Mass Air Flow Sensor w/ spray. Carefully removed MAF sensor, cleaned and reinstalled. CEL was off and idel went back to normal. For two days. Now light is back on, ideling low, wants to stall at stops. Any ideas that won't cost a bunch of money I don't have will be appreciated. Thanks.