losing www every few minuts

Hi, I'm using Mac Os X version 10.5.8 and have been losing my wireless connection every 5 to 35 minutes recently. Switching airport on and off used to help but doesn't do the job anymore, neither does network diagnosis trouble shouter.
After a (random) while www comes back just like that. very annoying.... It's not the modem, since my PC using fellow in the same room using the same wireless has a stable connection all the way.... Any ideas out there in the brave world??
I'd love a hint!
Jules

What kind of Mac is this? How long has this been going on? What kind of network is it? Depending on network and environment, you might be experiencing interference. How good is your signal strength to the router?

Ah, thanks, it's a MacBook, has been going on for a month or so but getting worse, it's a wireless tpg network in Australia, but I'm in Sydney, getting a very strong signal normally and It even doesn't help if I plug in the ehternet cable.