Aliased IPs

[redhat at buglecreek.com]

We have a server that that has a bunch of aliased IP addresses (eth0:1 -
eth0:20).  When doing a reboot or "service network restart" all of the
aliased ips show in the up condition with ifconfig, but when trying to
ping some of them they do not respond.  When we manually do ifup eth0:5
for example the interface starts responding.  If we do this to all the
interfaces that initially do not respond they all seem to work fine. 
Any clues as to why this is?  

System runs Redhat ES4 Update 2

Standard interface config file:
DEVICE=eth0:5
ONBOOT=yes
BOOTPROTO=static
IPADDR=xxx.xxx.xxx.xxx
NETMASK=xxx.xxx.xxx.xxx
GATEWAY=xxx.xxx.xxx.xxx