2003 Ram 4.7L Why would the air work perfectly until stopping at a light and hear a little sound then starts blowing very hot air instead? Even when put in neutral and revved up doesn't help, until the truck is in motion again it will not get cold again. I've also noticed that every time the truck is turned off you can hear a hissing for maybe 6 seconds. I've drained all the refrigerant and pulled a vacuum and refilled it with top notch 134a and it's made no change except gets a little colder. The refrigerant never gets low either, what's causing this?