Roughrider, these wheels take prestas, it can't be a loose valve core. Maybe a bad valve, but really unlikely. D-roche, he's talking about a problem that can happen with schrader (car-style) valves.
What you have are slow leaks from small holes. You could probably find them if you held the tube under water for a while, but that's a job for a pretty boring saturday afternoon to save yourself $7 worth of tubes. I'd replace your tubes, but in the process I would pull your tires all the way off, get something sharp (good sharp ice pick, file a nail to a thin point, stick a thick needle in a cork, whatever), and go over each tire very carefully looking for bits of glass embedded in the tread. It's a bit hard to describe, but you want to kind of pinch the tread and stick your probe in every little cut you see; probably about 1/3 of them will have little glass shards. It's that stuff slowly working its way through the tire that is making the little holes. Repeat this process every so often and you will get a lot less flats; since I started being dilligent about this my only flats have been from obvious causes (ran over the bottom of a beer bottle, caught a nail, etc...). Also, when you do flat out, never put in a new tube until you've found the culprit and removed it from your tire.
Also, you don't really need those O-rings, and installing them before the tire is at least partly filled can result in pinching the tube under the bead of the tire.