First, you must verify the engine is getting hot enough. Check the heater hoses going into and coming out of the heater core. Make sure they are both very hot. If both are not hot, you may have a thermostat sticking open. If one is hot and the other is not, you may have a restricted heater core. If they are both hot, you may have a temperature control issue inside the car.
i bet the ac is not working or working but dies sometimes. the water pump is out side the engine so dont need to open the where the timing belt is. thats what good with the Ford ZX2 engine. I had the same problem when I bought my zx2 last year. This problem if the AC is not working changed the waterpump