packet forward stops time to time

Hello guys,
I'm using redhat 7.3(kernel 2.4.18), iptables 1.2.5
Since 4 years ago i installed this firewall. no any problem. Just since 1 month ago, it sometimes stop forwarding packets. but linux still works itself. After a while, it works again. How can I debug this problem? How can I monitor the status of packet fowarding?

thanks for any suggestion!!!

Best Regards/Fons