1995 Plymouth Neon Stalls In Hot Weather

I have a 1995 Plymouth Neon 2L SOHC engine with 296,000 KM that stalls when the daily temperature approaches 30 degrees Celsius. The car can be started by removing the gas cap and pushing in the gas vent door and then replacing the gas cap. But when you start to drive it looses power immediately and stalls. It is not until the daily temperature falls or the car is put inside out of the heat to cool down can the car be started and driven normally. I have spoken to a service technician at a local Chrysler dealership who indicated that unless the same conditions can be replicated that results in the car stalling, there isn't much they can do. The problem is that by the time you tow the car into the dealership, the car has cooled down and operates normally. Would you have any idea what would cause this to occur?