Hey JP ... Have you noticed how the temperature of the engine relates? Given the information you've given us here the first place I would look is the thermostat. After that you need to check the hoses leading to the heater core and the core itself. The fact it works at times still leads me to the thermostat however. I would also take a ride the library and see if they have the Chilton's for your model year. There will be a heating troubleshooting guide in it that will guide you towards the problem. Good luck!