I have a 97 glx with the duratec engine. I have replaced the thermostat,waterpump,coolant lines and heater core. Was fine for a few weeks now the temp guage fluctuates dramatically from mid to HOT! Majority of the time it blows very little hot air. If under acceleration or high rev when the gauge peaks out I then get hot air. I can hear "glugging" noises from heater core. Indicative of air but I have added coolant many times with the cap off to release any air. The car still runs fine no loss of power. All of the parts have less than 1500 miles on them. Any help would be greatly appreciated for I am at a loss.