My car has 190000 miles.
My car has an automatic transmission.
Short of a vacuum leak, this seems to indicate to a dirty or failing idle air control valve. As you know, the idle air control valve monitors the air intake as it is mixed with fuel prior to being injected into the engine at low speeds and at idle. This valve is controlled by the vehicle's computer which will adjust idle speed based upon other measurements such as engine temperature, intake air temperature, and electrical system load or voltage. When you accelerate, the engine RPM increases, and as you let off the gas, the RPM slowly returns to the normal idling speed.
This is all controlled by the idle air control valve. When the engine RPM drops below the normal range of 750-800 RPM (for most cars), this may indicate a dirty or faulty idle air control valve. In your case, your vehicle stalls instead of slowing to an idle the RPM's increase which indicates this valve is not working properly. To avoid any unnecessary repairs, I would recommend having an expert from YourMechanic come to your location to diagnose you stalling issue firsthand and repair your idle air control valve as necessary.
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