Using iStumbler, I have noticed that my MacBook Airport card will sporadically switch from a strong AP to a weak AP on the same network. I can manually change over to the strong AP via iStumbler, and watch the signal strength on the Airport icon go from one bar (i.e. the little dot) to four bars (the strong AP is right outside my door).
Any idea why the Airport card connects to a weaker Ap instead of a stronger one?