Server Crashing sort of?

I am running NT 4.0 SP6a with Exchange 5.5sp4. at least once a week the server fall off of the network unitl it gets rebooted. I have uninstalled all anitvirus software. nothing else is running. I have installed a nic card and new tcp/ip stack. Tcp/Ip is the only protocal being used. There is no errors in the event log. exchange acts and logs as if the server is still up and running. but no one can access it. and the performance monitor is running about 25% on CPU usage. when it goes down. Is there anything else that I can try? Any Suggestions would be helpful.