[CentOS] squid stops working several times a day

Hello,

I have a problem here with:

[root at ablprx01 squid]# cat /etc/*release
CentOS release 5.2 (Final)
[root at ablprx01 squid]# rpm -qa|grep -i squid
squid-2.6.STABLE6-5.el5_1.3

The web proxy process (used by 300-400 users)
seems to run ok:

[root at ablprx01 squid]# ps uawwwx|grep squid
root      2304  0.0  0.0   7792  1268 ?        Ss   08:45   0:00 squid -D
squid     3082 27.7  0.9  46912 37068 ?        R    10:21  32:18 (squid) -D
squid     3083  0.0  0.0   1508   232 ?        Ss   10:21   0:00 (unlinkd)

[root at ablprx01 squid]# netstat -an|grep LISTEN
tcp        0      0 0.0.0.0:8080                0.0.0.0:*
     LISTEN
tcp        0      0 0.0.0.0:3128                0.0.0.0:*
     LISTEN
.........

but periodically it will stop working
(web pages can't be displayed).

The "service squid restart" command
will display a lot of dots ..... but doesn't
help and I have to reboot.

I've checked /var/log/messages and also
/var/log/squid/* but don't see any warning there.

ifconfig doesn't report any errors:

[root at ablprx01 squid]# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:16:35:5B:39:AE
          inet addr:10.121.42.32  Bcast:10.121.255.255  Mask:255.255.0.0
          inet6 addr: fe80::216:35ff:fe5b:39ae/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:989588 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1150056 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:590019020 (562.6 MiB)  TX bytes:639895003 (610.2 MiB)
          Interrupt:185

and the disk space is plenty.

Has anybody had a similar problem and
how would you debug it please?

Thank you for any hints
Alex

PS: Below my config file:

[root at ablprx01 squid]# grep -v ^# /etc/squid/squid.conf | grep -v ^$
http_port 3128
http_port 8080
hierarchy_stoplist cgi-bin ?
acl QUERY urlpath_regex cgi-bin \?
cache deny QUERY
acl apache rep_header Server ^Apache
broken_vary_encoding allow apache
cache_dir ufs /var/spool/squid 40000 16 256
access_log /var/log/squid/access.log squid
dns_defnames on
refresh_pattern ^ftp:           1440    20%     10080
refresh_pattern ^gopher:        1440    0%      1440
refresh_pattern .               0       20%     4320
acl all src 0.0.0.0/0.0.0.0
acl manager proto cache_object
acl localhost src 127.0.0.1/255.255.255.255
acl to_localhost dst 127.0.0.0/8
acl SSL_ports port 443
acl Safe_ports port 80          # http
acl Safe_ports port 21          # ftp
acl Safe_ports port 443         # https
acl Safe_ports port 70          # gopher
acl Safe_ports port 210         # wais
acl Safe_ports port 1025-65535  # unregistered ports
acl Safe_ports port 280         # http-mgmt
acl Safe_ports port 488         # gss-http
acl Safe_ports port 591         # filemaker
acl Safe_ports port 777         # multiling http
acl CONNECT method CONNECT
http_access allow manager localhost
http_access deny manager
http_access deny !Safe_ports
http_access deny CONNECT !SSL_ports
acl our_networks src 10.121.0.0/16 172.25.0.0/16 10.122.1.0/24
http_access allow our_networks
http_access allow localhost
http_access deny all
http_reply_access allow all
icp_access allow all
cache_mgr it at XXXXXX.com
coredump_dir /var/spool/squid