89 Grand Am - Stalling In Hot Weather

I have an 89' Grand Am LE with the 2.5L 4-cyl. It seems to work great in except in the summer. On days of more than 80-85 degrees or so, it almost always quits after driving for a while in stop & go driving around town. For instance, after cruising down the highway at 70 Mph and coming to a stop light, or just around town at stop signs, it kills as u stop. After killing, there is absolutely no way to start it up again, but after waiting 5-10 minutes it fires right up and seems to run perfectly. The stall is often preceded by rough engine stumbling and misfires.

We've checked the fuel pump, changed the fuel filter, replaced all sensors onboard and the main computer chip for the car, double checked coolant levels and oil levels. The temperature gauge reads normal even at the time of the stall.

Any help/advice would be greatly appreciated. Thanks

HI cbc57, my grandmother used to have a car with the same engine. it would do the same thing as you explained (so did the cars of several of my friends). Seemingly what you described is the lock up torque on the transmission staying engaged. The lock up torque is almost like another speed for your transmission. What it actually does is loweres the rpm of your transmission to improve fuel economy and keep the transmission cooler. to prevent this from reoccuring is to un plug it. The lock up torque switch (sensor) is located on the left part of the transmission (section towards the nose of the car). Basically it looks like wires just connecting into the housing of the transmission. (bare all around and then the wires going into the middle of of the casing). you'll see it. hope its the right answer to your problem.
ps: if your transmission is a four speed, don't unplug it you'll burn the transmission but if its a three speed go ahead.