I got the thermostat from the gm dealer and he gave which ever one was recommended. i got the car good and warmed up today, and the heat gets hot if i hold the gas at about 3k rpms. as soon as i let off and let it idle the heat gets barly luke warm, when i rev it up again it gets hot right away. the temp gauge is just under the half way line on the temp guage. so the engine is good and warm. could my heater core be plugged?