2 things that it could be, one being a theory being that there is a very small tear in either the seal or something like that, and under 30psi, the elastic pressure of the rubber won't let air out, but over that pressure, the elastic pressure cant hold it. Other thing that it could be, is just the needle in the tire went bad. I had one go bad on a snow tire of mine. It would every once in a while not close all the way, and it would just piss out air. I would just keep hitting it with the air chuck until it stopped hissing.
Either way, you need a new valve stem