What wireless modes are you using?
2.4Ghz and 5Ghz routers: Try single mode G or mixed G and N on 2.4Ghz and single mode N on 5Ghz?
Channel Width set for Auto 20/40Mhz or try 20Mhz only.
Try setting a manual channel to a open or unused channel. 1, 6 or 11. 11 for single mode N if the channel is clear.
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.What wireless devices do you have connected?
Any other WiFi routers in the area? Link> Use
InSSIDer to find out. How many?
Try turning off
Short GI, WLAN Partition,and Extra Wireless Protection if you have it. Under Advanced/Advanced Wireless.
Enable
WMM Enable Under Advanced/Advanced Wireless.