Yup, this was a real common problem for GM. The problem is not with the down-shifting. Even if it got hung up in second or third gear, it's the fluid slippage in the torque converter that lets the engine run in gear while the car isn't moving. As far as that torque converter is concerned, it has no idea which gear the transmission is in when the car is standing still.
Because of that constant slippage in the torque converter, even though they become more efficient at higher speeds, the engine will always be running about 200 rpm higher than the transmission at highway speeds. That costs a small of amount of fuel mileage. To address that, Chrysler developed a lock-up clutch inside the torque converter for some '77 models. Today all the manufacturers do the same thing to increase fuel mileage. The original versions were totally mechanical and only unlocked below a certain speed or when in first or second gear. Today they are operated with an electric solenoid and some type of computer controls, and they will also unlock when you tap the brake pedal, release the throttle completely, or hit near wide-open-throttle.
What you will notice is the transmission will up-shift to second, then third gear, then, when the engine is warmed up and you're above a certain speed, typically 40 - 45 mph, engine speed will drop another 200 rpm when the torque converter locks up. That can feel like another up-shift in some cars.
The problem GM had was the torque converter would fail to unlock. That is what causes the engine stalling. Think of driving a car with a three-speed manual transmission. If you fail to press the clutch pedal when you come to a stop, the engine is going to stall. It doesn't matter if you're in first, second, or third gear. Also, it doesn't matter which gear you're in when you come to a stop; if the clutch pedal is pressed, the engine won't stall.
When this was a real common occurrence, many mechanics and car owners chose the inexpensive way out. That was to unplug the two-wire connector for the lock-up clutch solenoid. The torque converter would never lock up after that, so it couldn't get stuck and fail to release. When presented with the estimate for the proper repair, a lot of people were willing to put up with a loss of about two miles per gallon, so they'd just live without that feature.
To verify the lock-up clutch is working properly, once you know it's engaged at highway speed, hold the accelerator pedal and car speed perfectly steady, then briefly tap the brake pedal with your left foot. If you have a tach, you'll see engine speed increase 200 rpm for two or three seconds, then go right back down again when the clutch relocks. With no tach, you'll have to listen and feel the increase in engine speed.
You'll have to get an estimate from a transmission specialty shop to know what it would take to do the proper repair. If that is done, the problem was solved permanently.
Monday, June 22nd, 2015 AT 9:38 PM