iwi driver: timeout after 20 minutes of idle time

[Михаил Кипа]

There is FreeBSD8 system (cvsup`ed and recompiled kernel on PC-BSD system). Noteboot asus a3l with intel PRO wireless 2915 card. Wireless 802.11a network with WPA2 protection. I configure my system as following (rc.conf):
dhclient_flags="-c /etc/dhclient.conf"
synchronous_dhclient="YES"
ifconfig_rl0="DHCP"
ifconfig_iwi0="up"
wlans_iwi0="wlan0"
ifconfig_wlan0="WPA DHCP bmiss 200 roaming auto"
hostname="asus-a3l"

Problem: After long idle (about 50 minutes) iwi driver timed out without reason. If I have permanently working ping utility driver work well. But as I turn it off it timed out:
Mar 15 03:50:28 asus-a3l kernel: iwi0: timeout waiting for (null) firmware initialization to complete                        
Mar 15 03:50:28 asus-a3l kernel: iwi0: could not load boot firmware (null)                                                   
Mar 15 03:50:28 asus-a3l kernel: iwi0: timeout waiting for (null) firmware initialization to complete                        
Mar 15 03:50:28 asus-a3l kernel: iwi0: could not load boot firmware (null)                                                   
Mar 15 03:50:28 asus-a3l kernel: wlan0: Ethernet address: 00:12:f0:24:b2:fd                                                  
Mar 15 03:50:28 asus-a3l kernel: iwi0: need multicast update callback                                                        
Mar 15 03:50:28 asus-a3l last message repeated 6 times                                                                       
Mar 15 03:50:28 asus-a3l kernel: ipfw2 (+ipv6) initialized, divert loadable, nat loadable, rule-based forwarding disabled, default to deny, logging disabled                                                                                              
Mar 15 03:50:32 asus-a3l kernel: pcm0: <Intel ICH4 (82801DB)> port 0xe000-0xe0ff,0xe100-0xe13f at device 31.5 on pci0        
Mar 15 03:50:32 asus-a3l kernel: pcm0: [ITHREAD]                                                                             
Mar 15 03:50:32 asus-a3l kernel: pcm0: <Avance Logic ALC650 AC97 Codec>                                                      
Mar 15 03:50:34 asus-a3l ntpd[1588]: ntpd 4.2.4p5-a (1)                                                                      
Mar 15 03:50:42 asus-a3l avahi-daemon[1900]: WARNING: No NSS support for mDNS detected, consider installing nss-mdns!        
Mar 15 03:50:43 asus-a3l kernel: iwi0: need multicast update callback                                                        
Mar 15 03:50:50 asus-a3l kernel: drm0: <Intel i852GM/i855GM GMCH> on vgapci0                                                 
Mar 15 03:50:50 asus-a3l kernel: vgapci0: child drm0 requested pci_enable_busmaster                                          
Mar 15 03:50:50 asus-a3l kernel: info: [drm] AGP at 0xf0000000 128MB                                                         
Mar 15 03:50:50 asus-a3l kernel: info: [drm] Initialized i915 1.6.0 20080730                                                 
Mar 15 03:50:51 asus-a3l kernel: drm0: [ITHREAD]                                                                             
Mar 15 03:51:59 asus-a3l python: hp-systray[2605]: warning: No hp: or hpfax: devices found in any installed CUPS queue. Exiting.                                                                                                                          
Mar 15 04:16:17 asus-a3l wpa_supplicant[443]: WPA: Group rekeying completed with 00:24:01:0b:20:72 [GTK=TKIP]                
Mar 15 04:46:12 asus-a3l wpa_supplicant[443]: WPA: Group rekeying completed with 00:24:01:0b:20:72 [GTK=TKIP]                
Mar 15 05:16:08 asus-a3l wpa_supplicant[443]: WPA: Group rekeying completed with 00:24:01:0b:20:72 [GTK=TKIP]                
Mar 15 05:46:03 asus-a3l wpa_supplicant[443]: WPA: Group rekeying completed with 00:24:01:0b:20:72 [GTK=TKIP]                
Mar 15 06:15:59 asus-a3l wpa_supplicant[443]: WPA: Group rekeying completed with 00:24:01:0b:20:72 [GTK=TKIP]                
Mar 15 06:45:54 asus-a3l wpa_supplicant[443]: WPA: Group rekeying completed with 00:24:01:0b:20:72 [GTK=TKIP]                
Mar 15 07:15:50 asus-a3l wpa_supplicant[443]: WPA: Group rekeying completed with 00:24:01:0b:20:72 [GTK=TKIP]                
Mar 15 07:44:29 asus-a3l kernel: iwi0: device timeout

I tried to sysctl debug.iwi=2 but there is no information before first "iwi0: device timeout" message only after. What can I do to force my system works correctly?