Print Server Drops User-Connections

I have a print server running NT workstation, recently it has been dropping users from the network printers. I have replaced the NIC, increased the ram, and it is still happens. One message I receive is”RPC Server not available”, this is @ the client machine. A reboot usually brings brings the printers back on-line. Other times the printers will disappear completely. Using HP JetAdmin don’t find any errors there, except Server not found- but you can still do a test print to that particular printer.