Originally Posted by Longfemur

It's a little hard to diagnose the problem without being able to actually see the tube and tire. When you are removing the tire and tube, try to install and remove the tire and tube in such a way that when you take it off to repair the flat, and then you find the hole in the tube (by immersing the tube in a sink full of water for example), you can know where to look inside the tire and on the wheel. One thing you can do is to install the tires always the same way, like with the tire's label where the valve stem is and always on the same side of the bike. That gives you a good reference point. It does sound from your description as if you are getting a slow leak from something that is either in the tire or the rim.