System Error 51

Hello Everyone,
I am having a problem with a very large NT 4.0 SP6a server. Randomly it has begun to stop sharing to the network. This is a big problem since on the server all the user’s (1000) data (H: drive) is stored here (280gigs). I have found no indications in the event logs that there is a system problem. I can still do a “net view \\server” on the system and see the shares, but when I try to do a “net use j: \\server\share$” I receive the “System error 51. The remote computer is not available.” I thought at first that backups were eating to much scsi bandwidth, but this last time all backup jobs were turned off. The server is configured with and ATM NIC. I appreciate any help.