I've found this on syslog :

Apr 11 19:35:23 petite-chose kernel: [  611.992042] wlan0: deauthenticated from 00:25:15:3b:8e:50 (Reason: 7)
Apr 11 19:35:23 petite-chose NetworkManager[709]: <info> (wlan0): supplicant connection state:  associated -> disconnected
Apr 11 19:35:23 petite-chose NetworkManager[709]: <info> (wlan0): supplicant connection state:  disconnected -> scanning
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> (wlan0): supplicant connection state:  scanning -> associating
Apr 11 19:35:24 petite-chose kernel: [  613.245033] wlan0: authenticate with 00:25:15:3b:8e:50 (try 1)
Apr 11 19:35:24 petite-chose kernel: [  613.247476] wlan0: authenticated
Apr 11 19:35:24 petite-chose kernel: [  613.247584] wlan0: associate with 00:25:15:3b:8e:50 (try 1)
Apr 11 19:35:24 petite-chose kernel: [  613.249931] wlan0: RX AssocResp from 00:25:15:3b:8e:50 (capab=0x411 status=0 aid=1)
Apr 11 19:35:24 petite-chose kernel: [  613.249949] wlan0: associated
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> (wlan0): supplicant connection state:  associating -> associated
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> (wlan0): supplicant connection state:  associated -> 4-way handshake
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> (wlan0): supplicant connection state:  4-way handshake -> group handshake
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> (wlan0): supplicant connection state:  group handshake -> completed
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> Activation (wlan0/wireless) Stage 2 of 5 (Device Configure) successful.  Connected to wireless network 'NEUF_8E4C'.
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> Activation (wlan0) Stage 3 of 5 (IP Configure Start) scheduled.
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> Activation (wlan0) Stage 3 of 5 (IP Configure Start) started...
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> (wlan0): device state change: 5 -> 7 (reason 0)
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> Activation (wlan0) Beginning DHCPv4 transaction (timeout in 45 seconds)
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> Activation (wlan0) Stage 3 of 5 (IP Configure Start) complete.
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> (wlan0): DHCPv4 state changed nbi -> preinit
Apr 11 19:35:24 petite-chose dhclient: Listening on LPF/wlan0/00:22:43:6f:25:6b
Apr 11 19:35:24 petite-chose dhclient: Sending on   LPF/wlan0/00:22:43:6f:25:6b
Apr 11 19:35:24 petite-chose dhclient: DHCPREQUEST of 192.168.1.44 on wlan0 to 255.255.255.255 port 67
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> (wlan0): DHCPv4 state changed preinit -> reboot
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> Activation (wlan0) Stage 4 of 5 (IP4 Configure Get) scheduled...
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> Activation (wlan0) Stage 4 of 5 (IP4 Configure Get) started...
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> Activation (wlan0) Stage 5 of 5 (IP Configure Commit) scheduled...
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> Activation (wlan0) Stage 4 of 5 (IP4 Configure Get) complete.
Apr 11 19:35:24 petite-chose NetworkManager[709]: <info> Activation (wlan0) Stage 5 of 5 (IP Configure Commit) started...
Apr 11 19:35:24 petite-chose avahi-daemon[697]: Joining mDNS multicast group on interface wlan0.IPv4 with address 192.168.1.44.
Apr 11 19:35:24 petite-chose avahi-daemon[697]: New relevant interface wlan0.IPv4 for mDNS.
Apr 11 19:35:24 petite-chose avahi-daemon[697]: Registering new address record for 192.168.1.44 on wlan0.IPv4.
Apr 11 19:35:25 petite-chose NetworkManager[709]: <info> (wlan0): device state change: 7 -> 8 (reason 0)
Apr 11 19:35:25 petite-chose NetworkManager[709]: <info> Policy set 'Auto NEUF_8E4C' (wlan0) as default for IPv4 routing and DNS.
Apr 11 19:35:25 petite-chose NetworkManager[709]: <info> Activation (wlan0) successful, device activated.
Apr 11 19:35:25 petite-chose NetworkManager[709]: <info> Activation (wlan0) Stage 5 of 5 (IP Configure Commit) complete.
Apr 11 19:35:34 petite-chose ntpd[5669]: Listen normally on 3 wlan0 192.168.1.44 UDP 123
Apr 11 19:35:34 petite-chose ntpd[5669]: Listen normally on 4 wlan0 fe80::222:43ff:fe6f:256b UDP 123
Apr 11 19:36:16 petite-chose kernel: [  664.994392] wlan0: deauthenticated from 00:25:15:3b:8e:50 (Reason: 7)

As you could see 'wlan0: deauthenticated from 00:25:15:3b:8e:50 (Reason: 7)' seems to be my problem.
why this happens ?

Xavier.