I have a '07 Kia Spectra that while driving the heater works really great. If you stop the heater will cool off and blow cold air. If you rev the engine while sitting it will not blow hot. Checked the water and it is ok. There are no apparent leaks that I can find. It only started this weekend. Another thing is if you stop fast it cools off fast and if you stop slow it will cool off slow. It cools off no matter what even though as soon as you accelerate it will return to full force heat. Any ideas?