HP laserjet 600 M602 gets stuck Printing over WAN

Have been referred to post a question here from HP Technical support regarding a strange issue we have with the HP Laserjet 600 M602 Model Printer.

See below for a more detailed description of the problem:

Hi Din,

I have now completed the steps you suggested in our earlier phone conversation.
I have performed the Partial Clean and format disk and install of the firmware.
We have however still got the same issue.

To give you some more background information on our setup as it is quite bespoke.
We have a Database application which has been created in-house.

The database server is located in our datacentre with customers connecting to it from remote sites via tunnels to the datacentre created over the sites internet connection.

This runs on top of an application called Reality.
http://www.nps-reality.com/products

Reality acts as an interpreter between the database application and Linux.
This in turn is running on CentOS release 6.7 (Final) Linux Distribution.
Reality uses NPU to process print jobs from the database application.

"The Network Printing Utility (npu) is a network printing program which sends print jobs to printers connected across an Ethernet Local Area Network (LAN) using TCP/IP or OSI protocols, or across an X.25 Wide Area Network (WAN). In order to do this, npu makes use of the communications facilities provided as part of UNIX-Connect."

In the database application we use a custom PCL/PS driver that was written in house which provides the basic ability to print documents in the two printer languages.

When we print the documents from the database application most of the time it works fine.
However sometimes the jobs get stuck in the print queues within the database application.

When the job gets stuck it is usually part way through a document that is being printed.
The printer itself gives no indication of any error messages and just displays "ready" on the screen.
Printer error logs are also empty.

We are able to print jobs locally from windows at this point, but any further network print jobs from the server will not be processed by the printer.

To be able to print from the server to the printer we usually have to wait around 10-15 minutes before the printer we accept jobs from the server again.
We have found restarting the printer does not help at this point.

We have ruled out the NPU utility and our PCL / PS driver as being the cause of the issue by using the CUPS print server instead of NPU.

With CUPS we use the driver for the LaserJet 600 M602 which is provided in the hplips package.
http://hplipopensource.com/hplip-web/models/laserjet/hp_laserjet_600_m602.html

With CUPS the job is sent from our database application fine to the cups print server every time without fail.

If a job gets stuck when using cups, the web interface of CUPS will show a message of "Connecting to Printer"

Another peculiar occurrence we have found is that if we print from a site which uses a ADSL connection we notice the speed of jobs being printed out is slower and jobs get stuck.

If we print from a site which has a fibre connection, we notice the speed the jobs print out at is quicker and jobs do not get stuck.

The customer used to use the HP Laserjet 4345 MFP Model before the Laserjet 600 M602.
This was not an issue with the older model.

We have also tested using a Dell printer and again did not get the problem.

Sorry for the long email, hope this will help.