server going offline

I have a strange phenomenon going on the WinNT 4.0 server where my Exchange 5.5 is running seems to be disconnecting itself from the network. It does not write antyhing in the event viewer so I can troubleshoot it. Whern this happens I am not ableto even ping it by IP or UNC. If I reboot it come sback on. It is using a static IP. I have replaced the NIC to rule that out. Any one have any ideas?