My car stalls out when I change gears or idle too long. It starts fine, and will idle fairly well before I ever put it in gear, but has problems in gear and between gears.

One of the most common failures that can cause this problem that I have seen before is the Torque converter solenoid (TCC solenoid). This solenoid can stay stuck on and cause the torque converter to stay locked up, this will cause the vehicle to stall whenever it is put into gear. The vehicle will start and run fine in park or neutral but will stall out as soon as it is put into gear. This is the most common failure that can cause this.
Also a malfunctioning idle air control valve can cause this, if the valve isn't moving or responding to bump up the idle when put in gear it may cause the vehicle to stall out, the computer commands this valve to allow more air in to bump up the idle to compensate the load on theenginewhen put in gear, if it isn't moving or isnt moving fast enough the idle may be too low and could cause the engine to stall out.Have also seen bad crankshaft position sensor cause this ,as well as mass airflow sensor's ,TPS - throttle position sensor's . Your best bet , take it to a qualified repair an have it diagnosed . No guess work .

Possible torque converter clutch issue. Is the check engine light on? If so, have it scanned for transmission codes. If the torque converter clutch is locked up when it isn't supposed to be, the vehicle will stall. Another thing to check is for coking on the throttle plate. Remove the hose that goes from the air filter box to the throttle assembly and look inside the throttle bore to see if it is dirty/black. If so, use carb cleaner and a toothbrush to clean while holding throttle plate open and remove as much carbon deposits as possible. This can affect minimum idle airflow and also cause a stall

It can be low fuel pressure issue or clogged fuel pump. Or its engine related issues,---------------
Check the help link mentioned below to troubleshoot the problem: ------

Engine stalls, car stalls while starting, vehicle stalls while starting, Engine stalls and dies, Car stalls and stops,
http://alltypesofpartsreplacing.blogspot.com/2012/12/engine-stallscar-stalls-while.html

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These will help.
Thanks.

Engine stalls or RPM fluctuates at idle up and down or dies near idle.

This problem is commonly caused by a dirty automatic idle speed control valve and throttle valve but always run a OBD2 fault code as well as the cleaning procedure. Buy a can of throttle valve cleaner (do not use carburetor spray cleaner!) from NAPA or Carquest (made by CRC chemicals) and spray it into the air intake while the engine is running, use up about 1/2 the can, engine will try to stall hold the speed up, shut it down and let it soak for 30 minutes, restart and blow out the remaining fluid, shut it down and disconnect the negative battery cable for 5 Min's to reset the base idle control

check fuel presure

Try depressing slightly the gas pedal to see if the engine continues to run when you put it in gear. Might have to depredd gas pedal and brake simultaniusly. if the engine continues to run, then the problem is in the idle circuit. Try decarboning the throttle body and the idle speed control.

you need to adjust the engion timing

first try so SPT gas teatment may sure you have a half tank of gas and then put in a little more of gas if this help change the fuel filter also check the vaccum lines and make sure they are all on and in and good conditon and also checkout the intake manifold for leaks

This is a comon problem with GM cars.Most likely cause is the torqe converter clutch lock up solenoid.

Its possible it could be other things causing the problem, but seeing as you've checked most of the likely causes, it is most likely your Torque-Converter. The lockup clutch in your Stator-Fan located inside the Torque-Converter could be siezed, causing the car to stall.