Default Outbound IP Address With Virutal IPs

I have an interesting situation on my CentOS 4 server. I have a number of virtual adapters (e.g. eth0:1, eth0:2 etc), and for some reason all outbound traffic is going through one of these, not eth0. If I go to ipchicken.com, it shows the ipaddress of a virtual adapter, not the normal eth0 adapter.

I have been looking around for information on how to set the default adapter to eth0, but I can't seem to find anything. I can't reboot yet, as it's a production server, so I have to wait for 'scheduled maintenance'. If that's the solution, I can try it, but I am sure there is another underlying issue/routine here.

Is there a way, while live, to set the outbound adapter back to eth0?

Please note, that I have only just realized this has happened. In fact, it has been this way since November. I can tell because server name stamps on Email messages sent from this server indicate a virtual domain at that IP address. I recall I had done some work around that time, which had involved me ifup'ing and ifdown'ing some of these adapters (but not eth0, or the eth0:100 adapter (if I recall correctly) that is now the default).

Also note that I have groups of eth0 adapters, such like:
eth0:1
eth0:2
eth0:3
eth0:100
eth0:101
eth0:102

It's the eth0:100 adapter that is now the default.