Q: What's makes the temperature hand goes up then down?

If you are referring to the engine coolant temperature gauge on the instrument panel, unless the gauge is misreporting the temperature, a likely cause of too low of an engine temperature is a cooling system thermostat that is stuck in the full open position. If the thermostat is stuck open and the coolant is too low in temperature that would account for the inadequate heat in the cabin. There is also a heater control valve which you want to check and see if it is fully open, that is allowing hot engine coolant to enter the heater core (little radiator) in the passenger compartment. If you wish to schedule a heater is not working diagnostic please contact YourMechanic and they will dispatch a certified professional to assist you.