Try this. Make a file 8192cu.conf in directory /etc/modprobe.d/ with the command
sudo nano /etc/modprobe.d/8192cu.conf
and add the following lines
# Disable power management
options 8192cu rtw_power_mgnt=0 rtw_enusbss=0
I assume that you are using Edimax or NETIS wireless USB adapters.