I know what your problem is.
I had the same problem on the same wheels a while back. The holes kept appearing around the valve stem, at random times. Took me a while to figure it out.
Anyway, what's happening is that when you inflate your tubes, part of the tube gets "pulled under" itself around the valve stem (the abrupt change in thickness of the tube results in it happening at this specific point) and the pressure/friction ends up punching holes into the tube. Here's a rough illustration in MS Paint, the red circles are where the flats occur:
In order to prevent this from happening, you simply need to use the little nut for the valve stem that everyone generally throws away. Before you inflate your tires, tighten the nut onto the stem just enough to give it some preload. Then inflate the tires. After they are fully inflated, tighten the nut the rest of the way so that it is tight against the rim but don't overdo it. The nut will keep the tube, on the inside of the rim, tight against the rim so it doesn't flop over on itself.
Try this, let me know. I went through countless tubes before I figured this out, it was the worst.