Looking for help on my 91 testarossa. A/c works great and nice and cold but intermittantly all of a sudden heat starts coming out of the same vents. I shut it down and try it again them same thing... On it's own time or place. I'm Thinking the blend door is loosing vaccuum and letting hot air in or the water valve has a mind of it's own and letting hot water in??? I'm just guessing any help is appreciated! Thanks!