Intel Centrino Centrino Advanced-N 6235 weak wireless reception

I use this card in a computer with Ubuntu 12.04.3 installed. The wi-fi works, but the reception is very weak, compared to a Windows based pc at the exact same place (plus other issues like when external drive plugged, see my other post).

Do I need to install the linux driver for the card (http://wireless.kernel.org/en/users/Drivers/iwlwifi), or is it is already installed automatically by Ubuntu ?

Many Intel wireless devices are troubled by 802.11N implementation in some routers. I suggest you experimentally disable N capability. This is easily reversed if it turns out to be ineffective:

sudo -i
echo "options iwlwifi 11n_disable=1"  >>  /etc/modprobe.d/iwlwifi.conf
exit

Reboot and tell us your result.