NIC Card Just stops…..

I work for an ISP and we have a “unique” problem:

Mail Server with Asus MB and WIN 2000 Server running. The NIC card randomly stops working– we have tried replacing the card with 3 different brands ( 3com, Aopen, and Netgear) to no avail… Need help in a big way!!!! It appears the NIC will no longer recieve or transmit any packets resulting in our users not being able to connect to our mail server.. WIN 2K still functions fine, the only way to “revive” the NIC is to completely shut down the server and do a cold reboot HELP HELP HELP