Windows SBS2003 Drops Internet Connection

I administrate a small (1 server, <10 clients) network where I deployed a Windows Small Business Server 2003 Standard as domain controller, DNS / Exchange / WSUS / PostgreSQL / ... server, and default gateway (2 NICs). However the internet connection is occasionally dropped. There is no specific behaviour identifyable for it to occur - neither some specific action nor time (sometimes it happens after a few days, sometimes multiple times a day). After the connection has been dropped, someone must log into the server, disable the offending NIC, enable it again and everything is fine until the next failure. Alternatively one could restart the server, but it has the same effect and also disrupts internal activities. The internet provider (Inode) was not able to help, as they insisted their side of the network was working fine. The company urgently requests a fix as it is both cumbersome for them and there is only one person who has the login credentials to the server and knows how to administer the (current) fix.
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