Well, 5 seconds are maybe a little to fast to establish all those connections, and your server could be limited to 4 sessions from the same computer.
This line works for me with the lynx Textbrowser:
lynx -accept_all_cookies -dump http://www.whatever.com
Now you can parse the file for "Error", "404" etc.
There a a few more parameters for lynx, like source, auth.
Very intresting could it be, to measure the time it takes to get the page.