I had the same problem & turned out my rim tape was worn. I couldn't feel anything sharp, but apparently one of the nipples was wearing a hole through each tube. Very maddening, as I changed out tubes repeatedly & finally put a new tire on - same thing. I sheepishly took it to my LBS & they replaced the rim tape with something really cushy. No more flats (at least not from rim tape failure.) Shoulda figured - the rim tape was about 10 years old & had about 60,000 miles on it.