Have you contacted "isp.net", whoever they are, to make sure that they have not blackholed or tarpitted that particular machine's IP address for some reason?
He said that the box is behind a nat and only that box behind the nat has a problem. I doubt that it has been specifically targetted by ip.
I've narrowed things down to not include the Linux box. :)
I took the Linux box's IP and moved it to another non Linux machine. The same issues occur.
I have the network guy reviewing things.
Shawn