This question seems to be similar: My pc is disconnecting automatic from wi-fi and I have tried the solution offered:
Replace Network Manager with Wicd:
sudo apt-get install --reinstall wicd
sudo service network-manager stop
sudo apt-get remove --purge network-manager network-manager-gnome
sudo service wicd restart
And reboot your machine.
This solved the problem for me. Since then I've tried changing back to network-manager once, but then the connection problems returned immediately. Wicd has worked flawless ever since.
There was just one more problem to tackle. I use a Cisco VPN connection quite a lot and Wicd doesn't support that out of the box. So, I needed a vpnc client. Wicd is not as easy as Network-manager in that respect, but the following websites helped me getting that to work as well:
https://help.ubuntu.com/community/VPNClient
http://sourceforge.net/projects/vpnc-gui/
Hope this will help you as well.