So what exactly was the failure mode? Pinches near the valvestem? Or were they abrasion/punctures along the rim side of the tube? Have you verified your rimstrip is up to snuff? If there are exposed spoke nipple holes in your rim surface, get a Velox cloth rimstrip. If it's just smooth metal (a la any high-end Mavic Ksyrium or similar), check for burs. I've never heard of slightly undersized tubes failing because they were used in a 700x25 tube (until, perhaps now).

Perhaps the thicker rubber right near the valvestem didn't allow that portion of the tube to balloon enough which increased the stress on nearby rubber that was thinner like the rest of the tube.