Try this. Launch Terminal app and enter
sudo /usr/libexec/airportd debug +alluserland +alldriver +allvendor
sudo /usr/libexec/airportd logger
Now reproduce the issue and then paste the output of the Terminal app. here so we can try to decode it and help.
Another thing to check:
Make sure you use WPA2-Personal as your security method. If you use WPA-Personal, then the Mac won't connect in 11n mode. So set your wireless router to 802.11n and pick WPA2-PSK as the security.
On your Mac, delete the network from Preferred Network List's and then re-join using the WiFi Menu Extra. ( Scan and click on the network to join).