Internet sharing only works for a few minutes. URGENT

[spiri <spiri@netcabo.pt>]

Hi,
I have a debian sarge sharing the internet with W2K workstations

The server as two nics: eth0=LAN; eth1=internet.
It's as iptables running with a firestarter generated script
Is running dhcp-cliente.

Each worksation as a static IP and the gateway =server's IP
The DNS server are the ones from my ISP.

The problem is that the internet sharing only works for a few minutes, 
then all the network goes down,
in the workstatios and in the server, I can't ping nothing.

I done networking stop follow by networking start and everything starts 
working, until one of the workstations try to access the internet, then 
I have to restart the network.

I need to solve this problem urgently, your help is very welcome.
I post several forum but without results.

Thank you in advance.

regards,

spiri