port forwarding

I am trying to use iptables firewall in an environment where there is lot of incoming traffic destined to different web servers,ftp servers running on different machine in LAN. I have done this using port forwarding.

The problem I face with this is very frequently the rules go on and off. They do not work but are listed when iptables -L -n command is issued.

Can anyone please suggest any additional modules that I may need to use apart from ip_nat,ip_conntrack, ip_nat_ftp and ip_conntrack_ftp.

Thanks in advance.
Durga Prasad.