The Atheros Wireless Network card can be a bit of a problem to use on Ubuntu. The default network manager doesnt work all that good with the card, resulting in frequent disconnections. The best option is to install WICD network manager and remove the default manager. This worked like a breeze for me. I was using Linux Mint Helena. Basically the driver is getting confused while background scanning. In my home setup the Access Point is configured for both 802.11 modes B and G with the same essid for each. This is a problem with the network manager. WICD resolves this problem.