NAT keeps failing.

I have a problem that I've searched & searched for help on, to no avail.

Running Windows 2003 Server with 2 network cards. I have a static IP address
assigned by ISP which is on one network card. The other network card is fixed
at 192.168.0.1, and I have NAT configured to share the connection and the
server is running DHCP and DNS as well.

Everything works fine for a few minutes (sometime 2 mins, sometimes 10 mins)
and then NAT stops working. There are no error messages being logged anywhere
(but nothing goes through from the internal to the external network), and
Windows says the service is still running.

I restart the service and everything works fine for a few more minutes before
it stops again. I have tried different network cards, different server
hardware, different everything (except obviously I'm still using Windows 2003)
..

Does anyone have any ideas, because I'm plum out....


Mick