I have a Debian server in the form of a Raspberry Pi running Raspian. It has a USB Wi-Fi connection. Sometimes it would not respond when I SSH to it, and would require a reboot. I found something in syslog that may indicate what the problem is, can someone help with what this means?

Dec 16 15:34:17 raspberrypi wpa_supplicant[1501]: wlan0: WPA: Group rekeying completed with 00:21:29:6c:5c:3d [GTK=CCMP]
Dec 16 16:17:01 raspberrypi /USR/SBIN/CRON[2109]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Dec 16 16:34:17 raspberrypi wpa_supplicant[1501]: wlan0: WPA: Group rekeying completed with 00:21:29:6c:5c:3d [GTK=CCMP]
Dec 16 17:17:01 raspberrypi /USR/SBIN/CRON[2127]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Dec 16 17:34:17 raspberrypi wpa_supplicant[1501]: wlan0: WPA: Group rekeying completed with 00:21:29:6c:5c:3d [GTK=CCMP]
Dec 16 18:17:01 raspberrypi /USR/SBIN/CRON[2142]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Dec 16 18:34:17 raspberrypi wpa_supplicant[1501]: wlan0: WPA: Group rekeying completed with 00:21:29:6c:5c:3d [GTK=CCMP]
Dec 16 19:17:01 raspberrypi /USR/SBIN/CRON[2161]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Dec 16 19:31:29 raspberrypi kernel: [16615.391509] ieee80211 phy0: wlan0: No probe response from AP 00:21:29:6c:5c:3d after 500ms, disconnecting.
Dec 16 19:31:29 raspberrypi wpa_supplicant[1501]: wlan0: CTRL-EVENT-DISCONNECTED bssid=00:21:29:6c:5c:3d reason=4
Dec 16 19:31:29 raspberrypi kernel: [16615.416189] cfg80211: Calling CRDA to update world regulatory domain
Dec 16 19:31:30 raspberrypi ifplugd(wlan0)[1444]: Link beat lost.
Dec 16 19:31:40 raspberrypi ifplugd(wlan0)[1444]: Executing '/etc/ifplugd/ifplugd.action wlan0 down'.
Dec 16 19:31:40 raspberrypi wpa_supplicant[1501]: wlan0: CTRL-EVENT-TERMINATING - signal 15 received
Dec 16 19:31:40 raspberrypi ifplugd(wlan0)[1444]: Program executed successfully.
Dec 16 19:31:42 raspberrypi ntpd[1928]: Deleting interface #2 wlan0, 192.168.1.10#123, interface stats: received=321, sent=327, dropped=0, active_time=16596 secs
Dec 16 19:31:42 raspberrypi ntpd[1928]: 202.6.116.123 interface 192.168.1.10 -> (none)
Dec 16 19:31:42 raspberrypi ntpd[1928]: 203.99.128.34 interface 192.168.1.10 -> (none)
Dec 16 19:31:42 raspberrypi ntpd[1928]: 203.118.148.40 interface 192.168.1.10 -> (none)
Dec 16 19:31:42 raspberrypi ntpd[1928]: 202.89.49.65 interface 192.168.1.10 -> (none)
Dec 16 19:31:42 raspberrypi ntpd[1928]: peers refreshed