first got to say: credits to jac if this is your answer. I took it from this forum on a related question of the intel 5100 not connecting on N-speeds.
if you have Intel WiFi 5100 AGN and enable security WPA2-PSK AES and not running in N mode only max 54Mbps !!!! enable QoS on your router configuration
Did the trick for my 5100 and a zyxel nbg460N router. Hope it helps for you as well.
I've had a similar issue. Using the Intel WiFi 5100 (512AN_HMW) and the D-Link DIR-655. What did the trick for me was to enable WMM in the Advanced Wireless router configuration.
Log in DIR-655 administration, go to ADVANCED -> ADVANCED WIRELESS -> WMM Enabled. Check this box.