It could stall if you use the rear brake while going downhill. Is this the case?
Look for a vacuum leak.
Oxygen censor
A faulty PCV valve can certainly cause idling problems, especially after the engine has warmed.
The spark plug gap can cause the RPMs to fluctuate, while idling. The more probable cause of the fluctuating RPMs is a bad camshaft.
For starters change the fuel filter
You have a vacuum leak at the intake.
Spray down the throttle body outside to clean it & prevent anything from sticking there. Then while the engine is running spray inside intake as long as possible without stalling the engine. This should do it.
When a vehicle is idling, the engine may not produce enough heat to warm the coolant effectively, leading to cold air from the heater. In contrast, while driving, the engine works harder and generates more heat, allowing the heater to blow warm air. The engine temperature can rise while idling due to reduced airflow through the radiator and less coolant circulation, while driving increases airflow and coolant flow, helping to cool the engine.
The Idle Air Control valve located in the throttle body.
Engine bounce is a phenomenon where the engine speed rises rapidly and then drops back down while the vehicle is idling. This can be caused by issues with the fuel system, ignition system, or engine timing. It can lead to rough idling and poor engine performance.
Because the driver took their foot off of the gas and slowed down in a high gear. Or probably because they didnt push in the clutch. Either of those things can stall a manual transmission.
I had the same problem. It would stall out after coasting down a hill. Cleaned the throttle body about six months ago. Has not stalled since then.