I have removed all your imq devices from your post to make it shorter as they are not relevant at all.

I assume you are running the MultiWAN app here? Did you set up the virtual IP on eth3:200 manually, through the IP Settings screen or through the 1-to-1 NAT app?

You are trying to ftp from 192.168.50.70 to somewhere on the internet and it does not connect. When you change the server's LAN IP from 192.168.50.70 and it connects, what are you changing it to?
What is the result of:

arping -I eth5 192.168.50.70

ctl+c to stop it.

[edit]
You should only get one device replying.
[/edit]

Deactivate 1-to-1 NAT .

now working.

Thanks

Please give details. What is your ClearOS LAN IP and FTP client IP when it is working and when it is not working.
Are you using a single external IP or something like 1-to-1 NAT?
What do you get from:

grep IF /etc/clearos/network.conf

ifconfig | grep '^\S' -A 1