I have this problem with madwifi for several months now:
While connection generally works fine, every maybe two or three minutes it slows down dramatically. With iwconfig I can then confirm that the txpower has set itself down to 8, instead of 15 which seems to be the default (germany). With "iwconfig ath0 txpower 15" I can set it back and the connection is fast again, for another two minutes ... I don't know what causes this, it's definitively no system power daemon or something like that, since I tried several linux distributions, and also turned iwconfig's power management off. I remember that when I got this laptop in february, the problem wasn't there, but now is since about april or may.