What Hardware version is your router? Look at sticker under router.
What Firmware version is currently loaded? Found on routers web page under status.
What region are you located?
Are you wired or wireless connected to the router?
Some things to try:
What wireless modes are you using? Under Setup/Wireless/Manual.
Try single mode G or mixed G and N?
Channel Width set for Auto 20/40Mhz or try 20Mhz only.
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.