What is the problem when your car keeps running hot and you have replaced the water pump and thermostat?
ALSO, Keep in mind that your coolant system is designed to be pressurized. When it is under pressure the coolant can work very efficiently to keep your engine at operating temp. If you have a small leak, even a pinhole (or head gasket leak etc.) your engine will easyily overheat because the "boiling temperature" of the coolant is significantly decreased when its not pressurized. For example when the coolant is pressurized it can withstand about 270 degrees, but when not pressurized it can maybe withstand 230 degrees.