Just to update my situation with these accursed valve problems, I've experienced THREE additional failures since I last posted. Two were Vittoria Rubinos of the most recent variety and yesterday a Continental Gatorskin underwent the same failure. I had two other Gatorskins fail the same way in the past several years, and was sort of expecting this tire not to last. It turned out to be good for only a couple of weeks, essentially still new.

I remain at a complete loss to explain why this happens to me so often. It is a far more frequent problem than actual punctures. Nothing seems to help prevent it. Certainly not sealant or any method of mounting the tire. It is giving me excellent reasons to abandon tubulars, which I love for their ride qualities and tool-less ease of roadside replacement.

If anyone has a possible theory to a likely cause or prevention not already mentioned, please post your thoughts.