Home Server - Broken communication

[atica]

Have a home network - 4 PCs through a D-Link704P Router using ADSL internet access.
One of my PC acts as a server (Win2000) - as file and print server and for running a private web/ftp site (when I open the ports on the router). And at times it is used to surf the internet.
The problem I have - after a few hours (~3-4h) it stops being able to access the internet through the router and I have to reboot.
I do not have this problem with the other PCs which are always connected (one XP and two Win98 laptops).
I ran "Network Monitor" on the Win2K server - communication with the other PCs is OK.
I changed the cable to the router, and changed port on the router. No difference. Only the top indicator light is green on the router, the bottom is not lit.
I tested the router, it is OK.
I checked the ethernet card on the problem PC. OK.

Any idea anyone ?

Thank you

 

[atica]

.. and ...

- all private network IP addresses used are FIXED addresses. DHCP is turned off. I have been running successfully this configuration for 2 years.

Thank you for helping anyone.