I installed ubuntu-server 12.04 on my PC , and I use an usb wireless card to join the network.

It works ok when I boot up my PC , but the wireless disconnects after a while. I pkill wpa_supplicant and reload the driver rtl8192cu , then it works a again. Then it disconnect again after about a random minutes.
Here is the syslog:

22384 May 29 21:49:27 homecenter kernel: [ 6450.459313] wlan1: authenticated
22385 May 29 21:49:27 homecenter kernel: [ 6450.459535] wlan1: associate with f4:ec:38:45:62:74 (try 1)
22386 May 29 21:49:27 homecenter kernel: [ 6450.469080] wlan1: RX AssocResp from f4:ec:38:45:62:74 (capab=0      x431 status=0 aid=3)
22387 May 29 21:49:27 homecenter kernel: [ 6450.469085] wlan1: associated
22388 May 29 21:49:27 homecenter wpa_supplicant[2342]: Associated with f4:ec:38:45:62:74
22389 May 29 21:49:27 homecenter kernel: [ 6450.481933] ADDRCONF(NETDEV_CHANGE): wlan1: link becomes ready
22390 May 29 21:49:27 homecenter wpa_supplicant[2342]: WPA: Key negotiation completed with f4:ec:38:45:62:7      4 [PTK=CCMP GTK=CCMP]
22391 May 29 21:49:27 homecenter wpa_supplicant[2342]: CTRL-EVENT-CONNECTED - Connection to f4:ec:38:45:62:      74 completed (auth) [id=0 id_str=]
22392 May 29 21:49:38 homecenter kernel: [ 6461.472014] wlan1: no IPv6 routers present
22393 May 29 21:49:38 homecenter ntpdate[2263]: step time server 91.189.94.4 offset 0.012758 sec
22394 May 29 21:49:51 homecenter ntpdate[2404]: step time server 91.189.94.4 offset -0.001190 sec
22395 May 29 21:54:38 homecenter kernel: [ 6762.052030] wlan1: deauthenticated from f4:ec:38:45:62:74 (Reas      on: 2)
22396 May 29 21:54:38 homecenter wpa_supplicant[2342]: CTRL-EVENT-DISCONNECTED bssid=f4:ec:38:45:62:74 reas      on=2
22397 May 29 21:54:38 homecenter kernel: [ 6762.064744] cfg80211: All devices are disconnected, going to re      store regulatory settings
22398 May 29 21:54:38 homecenter kernel: [ 6762.064752] cfg80211: Restoring regulatory settings
22399 May 29 21:54:38 homecenter kernel: [ 6762.064757] cfg80211: Calling CRDA to update world regulatory d      omain
22400 May 29 21:54:38 homecenter kernel: [ 6762.069938] cfg80211: Ignoring regulatory request Set by core s      ince the driver uses its own custom regulatory domain
22401 May 29 21:54:38 homecenter kernel: [ 6762.069943] cfg80211: World regulatory domain updated: 
22402 May 29 21:54:38 homecenter kernel: [ 6762.069945] cfg80211:     (start_freq - end_freq @ bandwidth),       (max_antenna_gain, max_eirp)
22403 May 29 21:54:38 homecenter kernel: [ 6762.069949] cfg80211:     (2402000 KHz - 2472000 KHz @ 40000 KH      z), (300 mBi, 2000 mBm)
22404 May 29 21:54:38 homecenter kernel: [ 6762.069952] cfg80211:     (2457000 KHz - 2482000 KHz @ 20000 KH      z), (300 mBi, 2000 mBm)
22405 May 29 21:54:38 homecenter kernel: [ 6762.069956] cfg80211:     (2474000 KHz - 2494000 KHz @ 20000 KH      z), (300 mBi, 2000 mBm)
22406 May 29 21:54:38 homecenter kernel: [ 6762.069959] cfg80211:     (5170000 KHz - 5250000 KHz @ 40000 KH      z), (300 mBi, 2000 mBm)
22407 May 29 21:54:38 homecenter kernel: [ 6762.069962] cfg80211:     (5735000 KHz - 5835000 KHz @ 40000 KH      z), (300 mBi, 2000 mBm)