wrong return address

Hello

I seem to have an interesting problem which occurs after a period of
normal operation.

I have a DHCP server which serves out IP addresses reserved by MAC
addresses in one subnet and a dynamic range in private subnet for
machines that do not have a MAC address reservation. The first subnet
is the normal operating one and is routable from the other subnets in
the company. The second subnet is only used for installs and is not
routable from other subnets. The two subnets are on eth0 and eth0:0
respectively, the default gateway is on the first subnet ie.: via eth0.
This works fine for the most part but after a few months of operation,
we mysteriously loose connectivity from the other vlans in the company
to the DHCP server, the logs on the default gateway which is an iptables
firewall show that the packets coming from the DHCP server has the
return address of the IP aliased interface eth0:0 instead of the eth0's
IP address which it should have. ifdown'ing the the eth0:0 interface
and ifup'ing it again fixes the problem for another few months.

The distro is RH7.1 but with a 2.4.24 kernel.org kernel.

Any idea as to why packets would suddenly start using the wrong return
address?

Regards,
John