I faced a similar problem as well when I (clean) installed Mint 9; my Ralink RT2860 wireless card that was working perfectly in Mint 8/Karmic had a difficult time connecting and maintaining a solid connection wirelessly, in particular with networks with WPA. It was supposedly due to a kernel regression, so I updated my kernel to 2.6.33 (Mint 9/Lucid uses 2.6.32 by default) and wireless works perfectly now. The other alternative for me was to download the drivers from Ralink, and compile and install them myself, and I hear that several other Ubuntu users with the same wireless chipset used this method successfully, but downloading Ubuntu's mainline 2.6.33 kernel was much more faster and convenient...what can I say, I'm a lazy guy.
If nothing else works, consider going to Atheros' website and download the appropriate driver for your card, and compile and install manually following instructions normally provided in a readme file in the download. I mean, if drivers gotten directly from the manufacturer don't work, then I don't know what else could possibly work instead.