I do encounter the same problem here with xubuntu 15.04 on my Dell Latitude 430 quite frequently. 

Looking at my syslog messages, this situation always seems to start with the following output:
Jun  6 20:35:39 smithers kernel: [ 4126.496072] iwl3945 0000:0c:00.0: Error sending C_POWER_TBL: time out after 500ms.
Jun  6 20:35:39 smithers kernel: [ 4126.496086] iwl3945 0000:0c:00.0: set power fail, ret = -110
Jun  6 20:35:41 smithers kernel: [ 4128.472071] iwl3945 0000:0c:00.0: Queue 4 stuck for 2500 ms.
Jun  6 20:35:41 smithers kernel: [ 4128.472082] iwl3945 0000:0c:00.0: On demand firmware reload
Jun  6 20:35:41 smithers kernel: [ 4128.508118] ieee80211 phy0: Hardware restart was requested
Jun  6 20:35:41 smithers kernel: [ 4128.560704] iwl3945 0000:0c:00.0: BSM uCode verification failed at addr 0x00003800+0 (of 900), is 0xa5a5a5a2, s/b 0xf802020
Jun  6 20:35:41 smithers kernel: [ 4128.560715] iwl3945 0000:0c:00.0: Unable to set up bootstrap uCode: -5
      [ the last two lines are repeated 5 times ]
Jun  6 20:35:41 smithers kernel: [ 4128.786187] iwl3945 0000:0c:00.0: Unable to initialize device after 5 attempts.
Jun  6 20:35:41 smithers kernel: [ 4128.786198] ------------[ cut here ]------------
Jun  6 20:35:41 smithers kernel: [ 4128.786267] WARNING: CPU: 0 PID: 2505 at /build/buildd/linux-3.19.0/net/mac80211/util.c:1774 ieee80211_reconfig+0x47d/0x1480 [mac80211]()
Jun  6 20:35:41 smithers kernel: [ 4128.786271] Hardware became unavailable during restart.
Jun  6 20:35:41 smithers kernel: [ 4128.786275] Modules linked in: [ removed to shorten post ]
Jun  6 20:35:41 smithers kernel: [ 4128.786389] CPU: 0 PID: 2505 Comm: kworker/0:0 Not tainted 3.19.0-18-generic #18-Ubuntu
Jun  6 20:35:41 smithers kernel: [ 4128.786393] Hardware name: Dell Inc. Latitude D430                   /0XW631, BIOS A00 05/21/2007
Jun  6 20:35:41 smithers kernel: [ 4128.786419] Workqueue: events ieee80211_restart_work [mac80211]
     [ register dump removed to shorten post ]
Jun  6 20:35:41 smithers kernel: [ 4128.786457] Call Trace:
     [ call trace removed to shorten post]

     [ A really large number of such dumps follows ]

Jun  6 20:35:42 smithers kernel: [ 4128.803212] ---[ end trace eb90e628c7fb6207 ]---
Jun  6 20:35:42 smithers kernel: [ 4128.804736] cfg80211: Calling CRDA to update world regulatory domain
Jun  6 20:35:44 smithers kernel: [ 4131.825741] cfg80211: World regulatory domain updated:
Jun  6 20:35:44 smithers kernel: [ 4131.825748] cfg80211:  DFS Master region: unset
Jun  6 20:35:44 smithers kernel: [ 4131.825753] cfg80211:   (start_freq - end_freq @ bandwidth), (max_antenna_gain, max_eirp), (dfs_cac_time)
Jun  6 20:35:44 smithers kernel: [ 4131.825759] cfg80211:   (2402000 KHz - 2472000 KHz @ 40000 KHz), (300 mBi, 2000 mBm), (N/A)
Jun  6 20:35:44 smithers kernel: [ 4131.825764] cfg80211:   (2457000 KHz - 2482000 KHz @ 40000 KHz), (300 mBi, 2000 mBm), (N/A)
Jun  6 20:35:44 smithers kernel: [ 4131.825769] cfg80211:   (2474000 KHz - 2494000 KHz @ 20000 KHz), (300 mBi, 2000 mBm), (N/A)
Jun  6 20:35:44 smithers kernel: [ 4131.825774] cfg80211:   (5170000 KHz - 5250000 KHz @ 40000 KHz), (300 mBi, 2000 mBm), (N/A)
Jun  6 20:35:44 smithers kernel: [ 4131.825778] cfg80211:   (5735000 KHz - 5835000 KHz @ 40000 KHz), (300 mBi, 2000 mBm), (N/A)
Jun  6 20:35:45 smithers dhclient: receive_packet failed on wlan0: Network is down
Jun  6 20:31:40 smithers wpa_supplicant[1041]: message repeated 7 times: [ wlan0: WPA: Group rekeying completed with 64:7c:34:a6:33:e8 [GTK=TKIP]]
Jun  6 20:35:45 smithers wpa_supplicant[1041]: wlan0: CTRL-EVENT-DISCONNECTED bssid=64:7c:34:a6:33:e8 reason=3 locally_generated=1