I doubt this is a printer issue. It sounds to me like you are losing the connection between the JetDirect and your print server. You don’t mention what platform you are running for the corporate print server, or how that is set up. As a start I would remove this print server from the corporate one and set it up again using a new name. If you can copy the config of a working one during setup do that.
As a test to figure out if it is the printer or the print server/corporate connection, you could set up one of the local workstations to print directly to the print server via TCP/IP – LPR. This is pretty easy to do in Windows. Set up a printer to print to a local port, then create a standard tcp/ip port that points directly to the jet direct print server. That way you are not queueing through the corporate server. If that one prints then you know the problem is not with the printers.
Good luck – Doug