Sounds like you are an experienced rider, so you probably are not doing this, but just in case:
You don't have the nut on the presta valve tight up against the rim do you? It should only be up against the rim when inflating the tire, to keep from pushing the stem into the rim. Loosen it back up after inflating the tire. If it is tight while riding, it may not "give", causing excessive wear around the stem, causing a leak.