This happens with two different-vendor access points.
Internet disconnects while the nm-applet is showing connected.
Although, nm-applet shows that am connected and I got IP address but I even can't ping the router/my default gateway!!
So, I need to keep restarting the "/etc/init.d/network-manager" in order to work again just for a few minutes.
PS: The AP are working properly with my iPad, my Nexus 4 mobile and even with my brother's windows machine.
Relevant lspci output:
03:00.0 Network controller [0280]: Intel Corporation Centrino Wireless-N 1000 [8086:0084]
Subsystem: Intel Corporation Centrino Wireless-N 1000 BGN [8086:1315]
Kernel driver in use: iwlwifi
Kernel modules: iwlwifi
Kernel version: 3.2.0-37-generic