I have a 06 grand prix. the engine tempature guage was not running where it normally did which was about half way on the gauge. I put a thermostate in it, top off the fluid and bleed the system. the tempature guage now runs between the 1/4 line and half line on the guage which i think is normal. but the problem i am having now is, when the car is idleling after warmed up the heat cools way down. when i rev the engine and hold it about 3 grand it warms back up, then cools down unless the rpm's are up. is my heater core plugged?