Well, the MAS is the Mass Airflow Sensor. I'm not exactly sure the way Nissan likes to label it but if I recall in their explanation they quoted the "mass air sensor". Eithor way, this sensor controls the A/F (air to fuel) mixture ratio during acceleration and at idle. With other vehicles I've owned, the TPS or the Throttle Position Sensor controlled the idle so it threw me way off. Anyways, the MAS was calibrating the A/F too rich which was mainly noticed at idle -- resulting in engine stalls. My problem was very sporadic and sounds like your having the exact same symptoms I had. I've noticed a substantial increase in gas milage too.
If you wish, I have the receipt at home and can take a look at it to give you some part numbers. (if they're listed)
Hope this helps,
-Chris