OS X :: Connecting IMac To Internet Via AirPort

I bought a 27" iMac about two months ago, and recently it's been having trouble connecting to the internet via airport. Sometimes it has no trouble, other times, no matter how many times I put in my WEP password and hit connect, it just gives me the 'connection timeout error'. Basically, our whole household was having internet troubles about a month ago, but they mysteriously stopped for everyone else except me after a week or two. My housemates all have PC's, and are able to connect fine now. I also have a MacBook as well, which also has no trouble picking up the wireless. One of the weird quirks about this problem is that if my MacBook is connected and moved to sit right next to my iMac, I can often get the iMac to connect. And if it drops the connection, I can refresh a page on the MacBook and suddenly the internet works again on my iMac.

And I also discovered that if I move my iMac to the right about 30cms, it sometimes connects. This suggests that maybe I'm out of range for my wireless, but how am I able to connect at other times when the iMac in it's normal place? Anyway, I've ruled out a fault with the wireless itself, because, as I mentioned, every other computer can connect fine. I've also tried going into the system prefs and renewing the DHCP lease, which *sometimes* works, but usually doesn't. It has been connecting fine for about 2 days, but tonight it started acting up again. It's connected now, but it's pretty sporadic and keeps dropping the connection. In case you need to know, it's one of the new 27" iMacs, the usual duo core not the quad core. 4gb memory, etc.