I wrote my above while the other statements were coming in, so read it in that light. I'll leave it in case anyone else finds it of interest.

What tubes are you using? I have had a bad batch of tubes before that behaved the same way, the rubber behaved as if it were more brittle. A little patch was disintegrated. Just an idea to try a different tube if you are using lightweight stuff.

I assume the tube fits in nicely, no folds and not too tight? This is not a tube meant for a 28-32mm tire in a 19mm tire? Do you hear any creaking, like leather on leather, when you inflate the tube?