About 2 months ago, my radiator blew and was replaced (thank you extended warranty!) But now, my heat only blows warm/hot air when the car is moving. If it is idling in park, to warm up; it blows cold air. Also, when I am driving and there is heat, if I stop at a red light, it blows cold air. Mechanics can't find what is wrong and said they have to "research it". Any ideas?