06 Altima strange tire/wheel problem

Hi all:
I have an 06 Altima, 16k miles, with a strange problem. The left front tire goes flat intermittently. It has done this five times in the last three months. The first time it happened, I took the wheel/tire (all factory original) off and placed the spare on. I filled the tire to the proper pressure and set it aside in the garage for week. No pressure loss. It happened recently just before I had an appointment at the local dealer. I had them check it out this time. They reset the bead, checked for any nails, screws etc. and filled the tire, leaving over the weekend. No pressure loss again. I have soaped the tire, valve stem and wheel. No evidence of a leak. In desperation, I have started to note the position of the valve stem relative to the ground, thinking there may be a minute crack in the wheel. I would think that something is affecting the tire/wheel only when it is loaded with the weight of the car on it.
Any ideas? Has anyone had a problem like this?
Thanks
Jim