What Hardware version is your modem? Look at sticker under modem.
version is currently loaded? Found on modem web page under status.
What region are you located?
What wireless modes are you using? Under Setup/Wireless/Manual.
Try single mode G or N or mixed G and N?
Channel Width set for Auto 20/40Mhz or try 20Mhz only.
Try setting a manual channel to a open or unused channel.
What security mode are you using? Preferred security is WPA-Personal. WPA2/AES Only. Some WiFi adapters don't support AES, so you might want to try TPIK only or Auto.
Any other WiFi routers in the area? Use InSSIDer
to find out.