1st. tire change the stem was replaced with a 90 degree and went bad about 300 mi. later. The "stem keeper" was missing thus I keep a supply on hand just in case though I'd say the stem was defective but...
It sounds like you are still using the OEM style ? With the solid metal ones you don't need the keepers. I highly recommend them. Willow says "friends don't let friends ride with OEM stems" (or something like that)