Losing the default gateway could be caused by the computer side or the router side. You may want to restart all network devices and verify they are using proper configuration in regards of IP addresses depending if it is DHCP or static. Please make sure you have the latest driver for the wireless adapter and latest firmware on the router.
The Windows* Event Viewer in Control Panel and Intel® Proset’s Wireless Event Viewer may help identifying the issue. In order to get to this last one you may open Intel® Proset Connection Utility and go to Tools, Intel® Wireless Troubleshooter, Wireless Event Viewer. When you have this windows opened you may enable all other options in the Driver Log tab by going into File, Settings.
The following article may help you further: Wireless Networking; How do I improve my 802.11n wireless performance?
The options you get from the Advanced tab are normal, in order to get 40 MHz band you will need to set it on Auto and set the router to 40 MHz.
In case the issue persists please provide the operating system, security encryption type used and if possible a copy of the log when the issue occurs.
Thanks, but how do I edit my router?
The answer to such question may vary depending on the AP’s brand and model. Please contact the router’s manufacturer or download its user’s manual.