A common problem on many General Motors cars is the Torque Converter Clutch fails to release and causes the car to stall when it comes to a stop or when started and put in gear. Most of the time it is a stuck Torque Converter Clutch (TCC) solenoid, but this is not the only cause of this problem. Generally, if you unplug the TCC solenoid at the transmission and the symptoms go away, you have found the problem. But sometimes this can be misleading because you don't know for sure if it's a bad solenoid, dirt in the valve body or a bad signal from the ECM. I would suspect you will find the solenoid is bad or has been unplugged.