Gateway Error 600 - End of File Error

When I run a client it launches fine and shows the correct data, but every few minutes a pop up window appears ( not the regular error window ) that has a LOGOUT button at the bottom of it and displays something like connection error, gateway error 600 End of File Error ( I don’t have exact message )
The error keeps appearing for approx 30 seconds then the window disappears and the system returns to normal.
Its like the client loses communication from the gateway for some time and then regains communication.
I appologise for the vagueness of the description, but I will try and get a screen capture of it.
The client PCs that this happens on are behind some sort of a firewall, I’m not sure if this is the cause or not, I’m going to do some testing from clients free of the firewall.
I was wondering if this error has been seen before and what the possible cause might be.

Is there a stacktrace on the details screen of the error? That would be helpful.
It sounds very much like a networking problem - maybe something as simple as a loose cable, bad switch, bad router, or routing loop?