What wheels are you using? I had a similar problem with Mavic Open Pros and it didn't matter what kind of tire or tube. Basically the design of the rim allowed the tube stem to flex enough to eventually create a small tear at the base of the stem. The solution was to take a one inch square of old tube, slice a small x in it and slide it down over the stem all the way to the base. This provides enough padding that I've not had a single instance of the same type failure since.
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A conclusion is the place where you got tired of thinking. - S. Wright
Favorite rides in the stable: Indy Fab CJ Ti - Colnago MXL - S-Works Roubaix - Habanero Team Issue - Jamis Eclipse carbon/831