The Idle Air Control valve located in the throttle body.
The spark plug gap can cause the RPMs to fluctuate, while idling. The more probable cause of the fluctuating RPMs is a bad camshaft.
It could stall if you use the rear brake while going downhill. Is this the case?
You have a vacuum leak at the intake.
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.
Have your sensors checked.
It depends on how long you are idling, or how fast you are going. If you know you are going to be idling for a long time, it's best just to put your car in park.
check your coolant level.
How to fold down rear seats in 1998 Saturn
If you land on Saturn you might sink down and fall.
set the idle down on the carb
It is more than likely a thermostat
If it blows warm while idling and cool as you accelerate down the road, the coolant is low and or doesn't have proper mixture for your avg. temperature.