Vapor lock

When gasoline overheats and boils inside the carburetor bowl or fuel pump of a hot engine, it ceases to flow. This can cause stalling or hard starting. This is called vapor lock, and it usually happens during hot weather. If a hot engine won't start, all you can do is let it sit and cool off. You should check the cooling system to see if anything is causing the engine to run unusually hot (a bad thermostat or cooling fan, for example).

Is there a pattern to the stalling? It could be a fuel pump going bad (a pressure test can usually tell this) or something as simple at an IAC (Idle Air Control) or TB (Throttle Body) full of carbon deposits causing the stall.