apt fails behind tranparent proxy

Hi, I have a debian lenny server that i use ass transparent proxy with 
webfilter. For this i use an iptables script, squid3 and squidguard. 
With lenny worstations apt-get update often fails but this never happens 
with squeeze workstations. All worstations get ipaddress and dns from 
dhcp server. The transparent proxy works as expected when browsing. The 
iptables script is from a howto so my question is if anything in the 
script explains the apt-get behavior.

The apt-get update errors are (from memory):
  could not resolve "debian-mirror-hostname"
  could not download "debian-mirror-hostname" Release.gpg

The sources.list contains several mirrors (backports, multimedia, 
security,,,) but only some fails but not the same everytime i run 
apt-get update.

#!/bin/sh
# squid server IP
SQUID_SERVER="192.168.1.1"
# Interface connected to Internet
INTERNET="eth0"
# Interface connected to LAN
LAN_IN="eth1"
# Squid port
SQUID_PORT="3128"
# DO NOT MODIFY BELOW
# Clean old firewall
iptables -F
iptables -X
iptables -t nat -F
iptables -t nat -X
iptables -t mangle -F
iptables -t mangle -X
# Load IPTABLES modules for NAT and IP conntrack support
modprobe ip_conntrack
modprobe ip_conntrack_ftp
# For win xp ftp client
#modprobe ip_nat_ftp
echo 1 > /proc/sys/net/ipv4/ip_forward
# Setting default filter policy
iptables -P INPUT DROP
iptables -P OUTPUT ACCEPT
# Unlimited access to loop back
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
# Allow UDP, DNS and Passive FTP
iptables -A INPUT -i $INTERNET -m state --state ESTABLISHED,RELATED -j 
ACCEPT
# set this system as a router for Rest of LAN
iptables --table nat --append POSTROUTING --out-interface $INTERNET -j 
MASQUERADE
iptables --append FORWARD --in-interface $LAN_IN -j ACCEPT
# unlimited access to LAN
iptables -A INPUT -i $LAN_IN -j ACCEPT
iptables -A OUTPUT -o $LAN_IN -j ACCEPT
# DNAT port 80 request comming from LAN systems to squid 3128 
($SQUID_PORT) aka transparent proxy
iptables -t nat -A PREROUTING -i $LAN_IN -p tcp --dport 80 -j DNAT --to 
$SQUID_SERVER:$SQUID_PORT
# if it is same system
iptables -t nat -A PREROUTING -i $INTERNET -p tcp --dport 80 -j REDIRECT 
--to-port $SQUID_PORT
# DROP everything and Log it
iptables -A INPUT -j LOG
iptables -A INPUT -j DROP