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.