I had the same problem with valve stems until I found the cure. It turned out the valve hole had rough metal edges after being drilled at the factory. I chamfered these edges slightly with a piece of sand paper. For extra insurance I cut out a small square, 25 x 25 mm, from an old tube, then cut a very small hole in the square which is then fitted over the valve stem. The small square provides an extra layer of rubber "cushion" at the hole. I've not had flats from the same cause.