Your temperature hand goes down when you turn on the heat?

More then likely, your thermostat is sticking. When this happens, your car tends to overheat. If you turn on you heat, it keeps your car from overheating for a little longer then if you left it off. If you see your car is starting to get hot, turn on your heater, but try to get to wherever you needed to go, or to the nearest trustworthy mechanic.