2003 Pontiac Grand Am 4 cyl Auto Engine Dies on Road

Coolant flush should have nothing to do with performance. But, if it helped, it helped. Sounds like your problem is two fold. 1- The most common cause for stalling at a light in a GM product is the "idle speed control" ISC. They usually just need to be removed and cleaned. A regular can of spray carb cleaner will usually do the trick.
2- Stalling going down the road could be several things. Clogged fuel filter, ignition module, fuel pump are possible causes.
When was your last tune up?
Do you have, or have you had, a check engine light? If yes, we would need to know the stored codes and their descriptions. Please advise.

Fords are prone to fuel pump overheat if the fuel level gets low but not GMs. Going back to the ISC unit. They can be carboned up at the pintel valve where thay enter the intake manifold. The valve actuator works normally so the ECM thinks the valve is working so it doesn't set a code. You may want topull this unit out and have a look see. Another possibility is the EGR valve hanging open. Both of these can cause stalling at low speeds and on decceleration.

The EGR valve ( exhaust gas recirculation ) valve is located on the intake manifold. It is usually attached to the exhaust system with a heavy metal tube.