the heater in my 03 1500 4x4 4.7L will not blow hot air... It will blow warm air (just enough to defrost the bottom part of the windshield)... if i turn the heater off for a couple minutes then turn it back on it will blow hotter air for about a minute then go back to warm air.. What could cause this, the truck is fully warmed up and the heater was working normal earlier..