I should have mentioned what kind of tubes I'm using.

They are Bontrager Superlights for 650C wheels.

Annie has a good point -- maybe it's a bad batch of tubes.

Anyway, my last two Superlight tubes are installed -- hope they last. After that, I'll see if I have better luck with the non-Superlight Bontragers (I have one in my seat bag.)

Thanks for sharing your ideas and experiences!

-- Melissa