1996 Cutlass Supreme Stalls

I have a 1996 Cutlass Supreme that has had a problem stalling since I bought it. The car stalls when it sits idle in traffic jams. I have had the coolant system checked, and the head gasket replaced. Also had the thermostat replaced. The engine doesn't get hot, it just stalls. After 10 minutes it will restart, but will stall again if stopped in traffic for a period of time. Does anyone have any ideas?

Check the crank position sensor. In my 1990 Cutlass Ciera it is mounted on a metallic bracked behind the pulley in the front of the engine.
The back of the pulley has two concentric metallic rings with periodic cuts. These cuts signal the position of the crank to the sensor. The sensor profile looks like an E such that the rings run between the horizontal lines of the E.
It began acting up after a small stone got lodged between the ring. The stone knocked on the sensor every time the the mounting bracket warmed up. The engine would stall and restart after a couple of minutes. As the exterior temperature increased, the problem occurred more frequently.

I hope this helps.