99 Chevy Tracker stalls when idling

Your problem sounds a lot like what some older Chrysler cars used to do when they had carburetors and chokes.
What would happen is that the choke would open a bit early and if the timing of that event coincided with a stop, since the engine was a bit lean at that point, it would stall. (otherwise you could drive right through it and never have a problem).
With an injected engine, the same fuel air ratio needs to be maintained, but it is done differently. In a carbureted engine limiting the air supply would richen up the mixture. In an injected engine, instead of limiting the air supply, more fuel is added instead.
This is controlled by the computer which reads the signal from the coolant temperature sensor and "knows when the engine is hot enough to stop adding fuel. If the coolant sensor is slightly out of range, you may get to a point where the computer "thinks" you are up to temp, but you are not.
So the conclusion would logically be to replace the temp sending unit...but not always. A thermostat that is not operating perfectly, a slight vacuum leak, a slight egr valve leak, a slight problem with the idle control motor, a glitch in the throttle position sensor or dirt on the wires inside the bore of the mass air flow sensor can also change the composition of the air fuel ratio, as can a bad spray pattern from an injector or two.
I'd begin by replacing the thermostat and temp sending unit, then follow up with careful testing of all the components I mentioned, using a haynes or chilton manual as a test guide.
One not factory recommended way of cleaning out the fuel system is to add 3oz of acetone per 10gal of fuel. Acetone will change the vaporization rate of the fuel and cleans as it passes through the system I've personally tested this and contrary to what has been said about damage it can cause, it has never caused a failure anywhere in any system I've used it in. (even at higher concentrations)
You asked for ideas...so I gave you a bunch of them.
Just don't go replacing everything without testing...that is a waste of time and can get really expensive.
Good luck!

I had same problem on mine 99 tracker. There is Idle Control Valve on the throttle body right below wide air hose, there is 2 8mm bolts on it, unplug it, unscrew it, take it out, and clean with dry towel, put back in - works like a champ now!