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If it occurs only at idle, that eliminates the crank position sensor and the fuel injection pump relay. Those two electrical parts and the old plastic radiator are about the only three things on a '91 that might ever leave you stranded.
Your friend refers to to the Mass Airflow Sensor which is down low on the driver side, in the big black tube that runs from the air cleaner back to the throttle body which is on the upper driver side of the engine. The mass airflow sensor measures air flow by monitoring the resistance of a hot wire as the air flows past it trying to cool it. It is expensive, but those tend to either work or not. They are rarely intermittent. You might pull off the electrical connector on that sensor and inspect it for corrosion.
Back to your problem -
1) beneath your intake manifold is an idle air control valve. It is an electronically controlled valve that meters your idle air flow, and hence, controls idle speed. It is about 4" long and has two 3/4" hoses running to it.
2) On your throttle body next to the wheel that is rotated by the cable that comes from your throttle is an idle position switch that tells the car when you take your foot completely off the gas.
There may be instructions somewhere on this forum for testing these two components rigorously. As a first step, I would clean and inspect them.
Take a screwdriver and dismantle all of the air ductwork from the mass airflow sensor back to the throttle body and idle air control valve.
Remove the idle air control valve and clean it with aerosol carburetor cleaner.
Clean the throttle butterfly with aerosol carburetor cleaner as well. It would be best to remove it do do this, if you are comfortable with removing all the cables and wires and putting them back on.
Turn the throttle wheel by hand. You should be able to hear the idle position switch click just as the throttle leaves its stop. If it clicks, it is probably OK.
Inspect all air tubes for cracks as you put everything back together.
Good luck!
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