What wireless modes are you using on the 5Ghz???
Trying Single mode N only on 5Ghz?
I presume the MacBook Air supports 5Ghz?
What security mode are you using? Preferred security is WPA-Personal. WPA2/Auto TPIK and AES.
Any other WiFi routers in the area? Use
InSSIDer to find out.
Turn off Short GI and Extra Wireless Protection if you have it. Under Advanced/Advanced Wireless.
What Hardware version is your router? Look at sticker under router.
What Firmware version is currently loaded? Found on routers web page under status.