Thanks to MacWorld magazine's Mobile Mac column (December 2007 issue), I was able to fix the issue. The blame goes to interference, as suspected. Since we live in Silicon valley, a lot of our neighbors has wifi internet connection, so they interfere with our own network. Whenever the Mac powers up it searches for the least crowded available channel,and since my hubby works at home and has around 5 desktops all connected in one broadband, I always get kicked out of the network so the Airport tries to look for a signal that I can connect to. Having the same problem? Check your airport menu, if its crowded with other network and you can download iStumbler to check on what's really going on.