Network problem isolated to this macbook air, where to look next?
I advise you to download and run iStumbler http://istumbler.net/ on your Mac.
Run it around the place where you want to work and at peek hours
of the day (where your neighbours are also using their Wi-Fi PC and
routers).

This run should give you a clear indication of which channels are
received at a high power level. You should absolutely avoid these
overloaded channels (most probably 1, 6 &amp; 11) and pick a channel which
is either free or received at the lowest signal level where and when you work.