[cutlassthatstalls]

ok im gonna give u alot to digest here. this problem could be electrical, these cars commonly have loose connections & bad grounding on the battery cables. my diagnosis though is that the solenoid in your transmission is sticking. i believe its called a tcc soleniod or ttc solenoid and its similar to a thermostat sorta. it seems its sticking open so that when your driving fast and then slow down, your transmission is not downshifting and this causes u to stall, much like trying to start a manual car in 5th gear. also if your car is slow going up hills a) drive it in 2nd for safety reasons when going uphill and b) get tour tourque clutch converter checked, because the is a very common culprit of that on this model car.
hope i helped cause i only really know these cars in depth.
peace.