I have a 1994 Mitsubishi Montero LS. 163,000 mi. 3.0l engine. When I first start my vehicle, it runs fine. After warming up, the idle is very low. It stalls when I come to a stop. I cleaned the mass air flow sensor with a special cleaner. The vehicle ran fine for about 20 minutes. Then it happened again. LOW IDLE. What to do?