From what I can gather, it's a vacuum leak in the lines somewhere. I have a '77 T-Bird and my dad bought a '78 T-Bird brand new from the factory in '77. He told me (just the other day, in fact) that his used to open up after they sat for a few hours even right after he got the car new. He ended up taking his car to the Ford dealership that he got it from and they told him that it was just a vacuum leak. They fixed it and it worked great for about two months, then it started happening again. He said that the '77-'79 Birds were bad about that anyway. Oh well, I guess it's just for looks. LOL