Network Card Problems

My company has a web server (Server 2003 Web Edition) with 3 network cards installed. Two of the cards are internal only with 10.0.5.0 addresses. The other card has a static public IP. For some reason the card 3 (public) loses connectivity. It’s not a set period of time either, varies. Sometimes it will stay up a day or two, other times it goes down after 3 hours. And when i say down i mean you cannot connect to it from the internet. If you look at the actual card both physically and from network connections everything looks fine. If you do a Repair or disable and enable you are able to connect again. Any ideas on what’s wrong? I’m lost!