ath0 goes down periodically

At home I have a FreeBSD 6-CURRENT machine with an ath card acting as an
access point.  Every once in a while, wireless traffic stops and I have
to log in to this machine and manually bounce the interface to get it
going again.

I noticed that when this sort of outage occurs I can't ping from the
hostap machine on the wifi interface.  It looks like the interface queue
is full. (?)

It just happened now so here are the diagnostics I can think of:

# ifconfig -v ath0
ath0: flags=8c43<UP,BROADCAST,RUNNING,OACTIVE,SIMPLEX,MULTICAST> mtu 1500
        inet 192.168.2.1 netmask 0xffffff00 broadcast 192.168.2.255
        inet6 fe80::209:5bff:fec8:6919%ath0 prefixlen 64 scopeid 0x3
        ether 00:09:5b:c8:69:19
        media: IEEE 802.11 Wireless Ethernet DS/11Mbps mode 11b <hostap>
        status: associated
        ssid ***** channel 6 (2437) bssid 00:09:5b:c8:69:19
        authmode SHARED privacy ON deftxkey 1
        wepkey 1:40-bit <**********> tx+rx+def
        powersavemode OFF powersavesleep 100 txpowmax 0 txpower 60
        rtsthreshold 2312 protmode CTS wme ssid SHOW apbridge dtimperiod 1
        bintval 100
        AC_BE cwmin  5 cwmax  7 aifs  3 txopLimit   0 -acm ack
              cwmin  5 cwmax 10 aifs  3 txopLimit   0 -acm
        AC_BK cwmin  5 cwmax 10 aifs  7 txopLimit   0 -acm ack
              cwmin  5 cwmax 10 aifs  7 txopLimit   0 -acm
        AC_VI cwmin  4 cwmax  5 aifs  1 txopLimit 188 -acm ack
              cwmin  4 cwmax  5 aifs  2 txopLimit 188 -acm
        AC_VO cwmin  3 cwmax  4 aifs  1 txopLimit 102 -acm ack
              cwmin  3 cwmax  4 aifs  2 txopLimit 102 -acm

# ping 192.168.2.1
PING 192.168.2.1 (192.168.2.1): 56 data bytes
64 bytes from 192.168.2.1: icmp_seq=0 ttl=64 time=0.191 ms
64 bytes from 192.168.2.1: icmp_seq=1 ttl=64 time=0.149 ms

# ping 192.168.2.2
PING 192.168.2.2 (192.168.2.2): 56 data bytes
ping: sendto: No buffer space available
ping: sendto: No buffer space available

/var/log/messages has a couple of recent lines saying
"kernel: ath0: device timeout"

The fix:
# ifconfig ath0 down
# ifconfig ath0 up
# ping 192.168.2.2
PING 192.168.2.2 (192.168.2.2): 56 data bytes
64 bytes from 192.168.2.2: icmp_seq=0 ttl=128 time=3.392 ms
64 bytes from 192.168.2.2: icmp_seq=1 ttl=128 time=0.886 ms

The card is a Netgear WG311T:
ath0: <Atheros 5212> mem 0xe8120000-0xe812ffff irq 11 at device 12.0 on pci0
ath0: Ethernet address: 00:09:5b:c8:69:19
ath0: mac 5.6 phy 4.1 radio 1.7

The kernel is a little old:
FreeBSD 6.0-CURRENT #4: Tue Apr  5 18:07:56 EST 2005

But all the relevant source files seem up to date:
# ident /boot/kernel/kernel | grep ath
     $FreeBSD: src/sys/dev/ath/ath_rate/sample/sample.c,v 1.8 2005/04/02 18:56:50 sam Exp $
     $FreeBSD: src/sys/dev/ath/if_ath.c,v 1.87 2005/04/04 02:34:14 sam Exp $
     $FreeBSD: src/sys/dev/ath/if_ath_pci.c,v 1.12 2005/03/05 19:06:12 imp Exp $

(Latest if_ath.c is 1.88, the changelog says "honor new
IEEE80211_KEY_GROUP key flag")

This happens periodically.  I can't reproduce it at will, but it will
happen again eventually.  Is there anything further I can do to help
diagnose (and hopefully fix) this problem?

--Emil