I've been using ultralight tubes for years now (various brands, although I agree those Continentals are the bomb) without any problems. I love the fact that they are small and fit easily in a jersey pocket. I use them in both my road and mountain bikes.

Have you checked your rims around the valve-stem hole? If you are having issues with the same area of the tube they might have a slight burr or perhaps the rim-strip has disloged. As annie mentioned, it could also be a bad "batch" of tubes. It happens