I have a 96 Grand cherokee with a 5.2. I have replaced the iac twice. each time it has remedied the problem with the stalling at stops at idle Then it returns after a few days. why?

You replace sparkplugs,plug wire,tps,map,cap and rotor.Thats good,If the vehicle runs and you come to a stop and it stalls,That is the IAC (Idle air control) you say it ran fine for a week now its acting up, Service the IAC or get it replaced.

did you clean out the throttle body? a dirty throttle body can cause hesitation and stalling at idle at times.

Idle air control module is a very probable cause... is your check engine light on? if so stop at your local parts store and have them scan it ( autozone is free). Add surging at idle and stalling in reverse and you named all symptoms of a bad IAC module.

clean the IAC it is all you need to do. Battery voltage drops just because when the engine stalls it kills the alternator. Engine stalls beause the Idle Air Control (IAC) valve is dirt clogged. Uninstall the IAC valve, and clean it using electronic contact cleaner (3M will do). No need to spend anything else.

check for air leak in the air intake.

FORD: 1995-96 CONTOUR, CROWN VICTORIA, ESCORT, TAURUS, THUNDERBIRD
1996 MUSTANG LINCOLN-MERCURY: 1995-96 CONTINENTAL, COUGAR, GRAND MARQUIS, MARK VIII, MYSTIQUE, SABLE, TOWN CAR, TRACER LIGHT TRUCK: 1995-96 AEROSTAR, BRONCO, ECONOLINE, F-150-350 SERIES, RANGER, WINDSTAR
1996 EXPLORER This TSB article is being republished in its entirety to include the 1995 4.6L Thunderbird/Cougar. ISSUE: After a 1-4 hour engine soak time, long crank times and/or long crank to start followed by a stall may occur on some vehicles. No further stalling or rough idle will occur after the engine is running. The long crank and/or stall may be due to the Idle Air Control (IAC) Valve sticking.
ACTION: Replace the IAC Valve with a revised IAC Valve if no Diagnostic Trouble Codes (DTCs) are present. Refer to the following Service Procedure for details.
SERVICE PROCEDURE

1. Perform normal diagnostics and correct any DTCs as required by referring to the appropriate year Powertrain Control/Emissions Diagnosis (PC/ED) Service Manual.
2. If no codes are present, replace the IAC Valve by referring to the appropriate model/year Service Manual, Section 03-14. Refer to the following Idle Air Control Valve Application Chart for proper IAC usage.
   
   IDLE AIR CONTROL VALVE APPLICATION CHART Model Year Vehicle Engine Revised Part Number PASSENGER CARS 1995 Escort/Tracer 1.9L F5DZ-9F715-BA 1996 Escort/Tracer 1.9L F6CZ-9F715-BB 1995 Contour/Mystique 2.5L F5RZ-9F715-AB 1996 Contour/Mystique 2.5L F6RZ-9F715-AA 1996 Taurus/Sable 3.0L 4-Valve F5DZ-9F715-DA 1995 Taurus/Sable 3.0L 2-Valve F5DZ-9F715-EA 1996 Taurus/Sable 3.0L 2-Valve F6DZ-9F715-DA 1995 Taurus SHO 3.2L F6DZ-9F715-CA 1996 Taurus SHO 3.4L F6LZ-9F715-AC 1995 Taurus/Sable 3.8L F68Z-9F715-EA 1995 Crown Victoria/Grand Marquis/Town Car 4.6L 2-Valve F5PZ-9F715-AA 1996 Crown Victoria/Grand Marquis/Town Car/Mustang 4.6L 2-Valve F6AZ-9F715-EB 1996 Mustang 4.6L 4-Valve F65Z-9F715-BC 1995-96 Continental 4.6L 4-Valve F5OZ-9F715-AB 1995 Mark VIII 4.6L 4-Valve F3LZ-9F715-AA 1996 Mark VIII 4.6L 4-Valve F6LZ-9F715-AC 1995 Thunderbird/Cougar 4.6L 2-Valve F5PZ-9F715-AA 1996 Thunderbird/Cougar 4.6L 2-Valve F6AZ-9F715-EB LIGHT TRUCKS 1995-96 Ranger 2.3L F5OZ-9F715-AB 1995-96 Ranger 3.0L F67Z-9F715-AA 1995 Aerostar 3.0L F68Z-9F715-DA 1996 Aerostar 3.0L F69Z-9F715-AA 1995-96 Windstar 3.0L F68Z-9F715-DA 1995 Windstar 3.8L F68Z-9F715-EA 1996 Windstar 3.8L F68Z-9F715-CB 1995-96 Econoline/F-150-350 Series/Bronco 4.9L F68Z-9F715-EA 1996 Explorer 5.0L F67Z-9F715-BA

The idle air control valve is located on the back of the throttle body. It has an oval electrical connector and two torx screws holding it in. Unplug the connector, remove the two screws then lightly tap the body with a soft faced mallet to loosen it from the body. 
Once you have it out and prior to installing a new valve clean the seat inside the throttle body where the idle control fits. This must be very clean! Installation is in the reverse order. 
 To save some cash, pull the valve out and clean it with throttle body cleaner, this usually fixes the problem for quite a while.
Good luck and hope this helps.

try replacing the idle air control motor or IAC for short. i actually had one at work today with your same complaint. and sure enough the valve was a carboned up and wouldnt work properly. and its nearly impossible to clean to the point where it will work properly. cost for the part at a dealer is about $115.

Here is the most common cause of surges and stalls and low idle, it is the idle speed control air-bypass valve and throttle valve (IAC for short), they get full of gunk over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

check your transmission fluid,the motor will stall when fluid is low