I come bearing good tidings. I am posting from an Acer Aspire 7540 running Karmic and with:
... the identical wireless chipset to yours. The connection is fast and stable - now. Like you with a default install the connection was weak and soon disconnected and wouldn't reconnect. All I did was to install the packages:
06:00.0 Network controller: Atheros Communications Inc. AR928X Wireless Network Adapter (PCI-Express) (rev 01)
You need to reboot and then you should be OK.
Edit: of course, you will probably want to use the wired connection to do the install of the modules. And you don't need the backports repository enabled. Although they have 'backports' in the package name, they're in the main repository.