Maybe you can try as suggested here; someone with the same wifi card has this problem and solved it switching off the power mode to the wifi:

iwconfig wlan0 | grep "Power Management" Power Management:off

If it’s set to “on” then there become my performance problems. To
change it on runtime you just have to do :

sudo iwconfig wlan0 power off

If you, like me, want to set it always off then you can for example
modify the script /usr/lib/pm-utils/power.d/wireless . I simply set
this on line 39 (it's iwlagn) : iwconfig_batt="power off"

http://ubuntuforums.org/archive/index.php/t-1845983.html