What wireless modes are you using? Try Single mode G or N or Mixed G and N on 2.4Ghz and Single mode N on 5Ghz?
What security modes? Try WPA2 and AES only at first, then switch to Auto or TPIK only?
What band are you using, Auto 20/40Mhz should work however try 20Mhz only to test.
Try setting a manual channel, Try 11, then 6, the 1. Also see if there is any near by routers near by using InSSIDer app. If there are many on 2.4Ghz, the router could be having problems contending with them in finding a clear and open channel.
Good info here:D-Link Main Router and AP Infomration