Hi, I have an Edimax EW-7811UN that I have exactly the same problem with, & I finally managed to fix it last week...

Create this file as root (or using sudo) with your text editor of choice

/etc/modprobe.d/8192cu.conf

paste these lines into it, then reboot.
CODE: SELECT ALL
# Prevent power down of Edimax WiFi Dongle when idle
options 8192cu rtw_power_mgnt=0 rtw_enusbss=0