Or your neighbor just got wireless and they are using the same channel or the channel next to the one you are using..
iwlist ath0 scan
- to check for other nearby access points.
It could be related to incorrect antenna settings since hte madwifi driver does NOT automatically detect this setting..
Take a look at UserDocs/AntennaDiversity.
You'll want to disable transmit diversity if you only have one antenna connected.
The other item in their FAQ suggests if you are having performance issue s that you get the latest version of the madwifi driver before asking questions through the support channels. http://madwifi-project.org/wiki/UserDocs/GettingMadwifi