I am using a Model B 256MB Pi with an Edimax EW-7711UTn USB WiFi adaptor. After some hours, the WiFi connection begins to drop and be restarted. Until it stops working completely. This is what I see in /var/log/messages:

    Mar  9 20:39:29 raspberrypi kernel: [ 9353.773167] cfg80211: Calling CRDA to update world regulatory domain
    Mar  9 20:39:29 raspberrypi wpa_action: WPA_IFACE=wlan0 WPA_ACTION=DISCONNECTED
    Mar  9 20:39:29 raspberrypi wpa_action: WPA_ID=0 WPA_ID_STR= WPA_CTRL_DIR=/var/run/wpa_supplicant
    Mar  9 20:39:29 raspberrypi wpa_action: ifdown wlan0
    Mar  9 20:39:30 raspberrypi wpa_action: removing sendsigs omission pidfile: /run/sendsigs.omit.d/wpasupplicant.wpa_supplicant.wlan0.pid
    Mar  9 20:39:33 raspberrypi kernel: [ 9357.478605] wlan0: authenticate with 30:46:9a:54:6f:3d
    Mar  9 20:39:33 raspberrypi kernel: [ 9357.624628] wlan0: send auth to xxx (try 1/3)
    Mar  9 20:39:33 raspberrypi kernel: [ 9357.626510] wlan0: authenticated
    Mar  9 20:39:33 raspberrypi kernel: [ 9357.703189] wlan0: associate with xxx (try 1/3)
    Mar  9 20:39:33 raspberrypi kernel: [ 9357.707045] wlan0: RX AssocResp from 30:46:9a:54:6f:3d (capab=0x411 status=0 aid=2)
    Mar  9 20:39:33 raspberrypi kernel: [ 9357.722534] wlan0: associated
    Mar  9 20:39:33 raspberrypi wpa_action: WPA_IFACE=wlan0 WPA_ACTION=CONNECTED
    Mar  9 20:39:33 raspberrypi wpa_action: WPA_ID=0 WPA_ID_STR= WPA_CTRL_DIR=/var/run/wpa_supplicant
    Mar  9 20:39:33 raspberrypi wpa_action: ifup wlan0=default
    Mar  9 20:39:40 raspberrypi wpa_action: creating sendsigs omission pidfile: /run/sendsigs.omit.d/wpasupplicant.wpa_supplicant.wlan0.pid
    Mar  9 20:39:40 raspberrypi wpa_action: bssid=xxx
    Mar  9 20:39:40 raspberrypi wpa_action: ssid=xxx
    Mar  9 20:39:40 raspberrypi wpa_action: id=0
    Mar  9 20:39:40 raspberrypi wpa_action: mode=station
    Mar  9 20:39:40 raspberrypi wpa_action: pairwise_cipher=CCMP
    Mar  9 20:39:40 raspberrypi wpa_action: group_cipher=CCMP
    Mar  9 20:39:40 raspberrypi wpa_action: key_mgmt=WPA2-PSK
    Mar  9 20:39:40 raspberrypi wpa_action: wpa_state=COMPLETED
    Mar  9 20:39:40 raspberrypi wpa_action: ip_address=xxx
    Mar  9 20:39:40 raspberrypi wpa_action: address=xxx

And dmesg sometimes tells me:

    [20014.112395] wlan0: deauthenticating from xxx by local choice (reason=3)

wpa processes running are:

     1872 ?        Ss     0:00 /sbin/wpa_supplicant -s -B -P /var/run/wpa_supplicant.wlan0.pid -i wlan0 -W -D nl80211,wext -c /etc/wpa_supplicant/wpa_supplicant.conf
     2010 ?        Ss     0:00 /sbin/wpa_cli -B -P /var/run/wpa_action.wlan0.pid -i wlan0 -p /var/run/wpa_supplicant -a /sbin/wpa_action


Does anyone else have these problems, and how can I fix those?

Since many people seem to have problems with insufficient power supplies: My RasPi runs on a 1.2A 5V supply and has a 3.8A 5V powered USB hub attached to it. I now detached all other devices from the powered hub and connected the Edimax to it, maybe this will help. 

Update: No, this time it took only a few minutes for the WiFi to become unresponsive.

Update 2: I still have problems, right now I noticed that the connection can be re-started using ifdown and ifup, but I keep getting more and more of these messages:

    Mar 22 19:11:59 raspberrypi kernel: [215435.967714] cfg80211: Calling CRDA to update world regulatory domain
    Mar 22 19:12:01 raspberrypi kernel: [215437.948971] wlan0: authenticate with XXXXXX
    Mar 22 19:12:02 raspberrypi kernel: [215438.087245] wlan0: send auth to XXXXXX (try 1/3)
    Mar 22 19:12:02 raspberrypi kernel: [215438.089006] wlan0: authenticated
    Mar 22 19:12:02 raspberrypi kernel: [215438.165765] wlan0: associate with XXXXXX (try 1/3)
    Mar 22 19:12:02 raspberrypi kernel: [215438.169604] wlan0: RX AssocResp from XXXXXX (capab=0x411 status=0 aid=1)
    Mar 22 19:12:02 raspberrypi kernel: [215438.185759] wlan0: associated

This happens in shorter and shorter timespans until I reboot. Then it's ok again for a day or two. So what's going on here?

Update: Ok, I just hooked the Pi up to a display, and started copying files via WiFi. After about 15 minutes the Pi just froze. I had a tmux running with output of htop and tail -f /var/log/messages. No visible kernel panic, nor anything in /var/log/messages. The machine is frozen.