Had the same problem running the following environment:

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Acer Extensa 5630 EZ

Atheros AR9280 MAC/BB Rev:2 AR5133 RF Rev:d0: mem=0xf9700000, irq=17

Linux 2.6.28-11-generic #41-Ubuntu SMP Wed Apr 8 04:38:53 UTC 2009 i686 GNU/Linux

Fritz Box 7270 using WPA2 encryption
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Problem was:
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Losing connection, even wlan icon shows no interrupt
Logfiles showed usually following entries:

Apr 10 22:54:48 nodename avahi-daemon[4715]: Registering new address record for fe80::224:2bff:fe89:6a4b on wlan0.*.
Apr 10 22:54:52 nodename NetworkManager: <info>  (wlan0): supplicant connection state change: 7 -> 6 
Apr 10 22:54:52 nodename NetworkManager: <info>  (wlan0): supplicant connection state change: 6 -> 7 
Apr 10 23:01:XX nodename NetworkManager: <debug> [12XXXX7299.414858] periodic_update(): Roamed from BSSID 00:XX:XX:XX:XX:XX (WLANname) to (none) ((none)) 
Apr 10 23:01:45 nodename NetworkManager: <debug> [12XXXX7305.418905] periodic_update(): Roamed from BSSID (none) ((none)) to 00:XX:XX:XX:XX:XX (WLANname) 
Apr 10 23:03:XX nodename NetworkManager: <debug> [12XXXX7419.486881] periodic_update(): Roamed from BSSID 00:XX:XX:XX:XX:XX (WLANname) to (none) ((none)) 
Apr 10 23:03:45 nodename NetworkManager: <debug> [12XXXX7425.491360] periodic_update(): Roamed from BSSID (none) ((none)) to 00:XX:XX:XX:XX:XX (WLANname) 
Apr 10 23:04:52 nodename NetworkManager: <info>  (wlan0): supplicant connection state change: 7 -> 6 
Apr 10 23:04:52 nodename NetworkManager: <info>  (wlan0): supplicant connection state change: 6 -> 7 
Apr 10 23:05:XX nodename NetworkManager: <debug> [12XXXX75XX.558856] periodic_update(): Roamed from BSSID 00:XX:XX:XX:XX:XX (WLANname) to (none) ((none)) 
Apr 10 23:05:45 nodename NetworkManager: <debug> [12XXXX7545.562906] periodic_update(): Roamed from BSSID (none) ((none)) to 00:XX:XX:XX:XX:XX (WLANname) 
Apr 10 23:07:XX nodename NetworkManager: <debug> [12XXXX7659.630870] periodic_update(): Roamed from BSSID 00:XX:XX:XX:XX:XX (WLANname) to (none) ((none)) 
Apr 10 23:07:45 nodename NetworkManager: <debug> [12XXXX7665.634905] periodic_update(): Roamed from BSSID (none) ((none)) to 00:XX:XX:XX:XX:XX (WLANname) 
Apr 10 23:09:XX nodename NetworkManager: <debug> [12XXXX7779.711850] periodic_update(): Roamed from BSSID 00:XX:XX:XX:XX:XX (WLANname) to (none) ((none)) 
Apr 10 23:09:45 nodename NetworkManager: <debug> [12XXXX7785.714892] periodic_update(): Roamed from BSSID (none) ((none)) to 00:XX:XX:XX:XX:XX (WLANname) 
Apr 10 23:11:XX nodename NetworkManager: <debug> [12XXXX7899.787846] periodic_update(): Roamed from BSSID 00:XX:XX:XX:XX:XX (WLANname) to (none) ((none)) 
Apr 10 23:11:45 nodename NetworkManager: <debug> [12XXXX7905.790908] periodic_update(): Roamed from BSSID (none) ((none)) to 00:XX:XX:XX:XX:XX (WLANname) 
Apr 10 23:13:XX nodename NetworkManager: <debug> [12XXXX8019.866865] periodic_update(): Roamed from BSSID 00:XX:XX:XX:XX:XX (WLANname) to (none) ((none)) 
Apr 10 23:13:45 nodename NetworkManager: <debug> [12XXXX8025.870886] periodic_update(): Roamed from BSSID (none) ((none)) to 00:XX:XX:XX:XX:XX (WLANname) 
Apr 10 23:14:52 nodename NetworkManager: <info>  (wlan0): supplicant connection state change: 7 -> 6 
Apr 10 23:14:52 nodename NetworkManager: <info>  (wlan0): supplicant connection state change: 6 -> 7 
Apr 10 23:15:XX nodename NetworkManager: <debug> [12XXXX81XX.946904] periodic_update(): Roamed from BSSID 00:XX:XX:XX:XX:XX (WLANname) to (none) ((none)) 

Tried the following:
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Changed driver to NDISWRAPPER -> no success
Change driver to ath_pci -> no success
Upgrade from 8.10 to 9.04 -> no success
Change from network mananger to wicd -> no success
Changed routers encryption from WPA2 to WPA -> success
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Network connection works fine now, but connection speed decreased to "just" 54m, before I had an unstabel 802.11n connection up to 260m