It sounds like others are experiencing the same thing I see just about everyday in my business concerning the valve stems. There are a lot of import valve stems, that aren't really holding up too well. Here is an article from one of my tire trade journals all the way back in 2001. Believe me, the problem has only gotten worse in the later years.
http://www.babcox.com/editorial/tr/tr20134.htm
I sorta remember talking about this before and went back and found this post from last year.
http://www.trailmanorowners.com/foru...9&postcount=11
Yes, Virgina Deacon, as far as the metal stems rubber grommet.....you can see it actually dry rot/crack when it gets old. You can also just replace the rubber piece, instead of the whole metal stem, when the time comes.