Help with Strange Python scraping error. HTTPError with one machine while it works on others.

Question

I am using a proxy and following is the code.

 20     req = urllib2.Request(url)
 21     # run the request for each proxy
 22     # now set the proxy
 23     req.set_proxy(proxy, "http")
 24     req.add_header('User-Agent', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.9.0.3) Gecko/2008092417 Firefox/3.0.3')
 25     req.add_header('Referer', 'http://www.google.com/')
 26     # time it
 27     start = time.time()
 28     # try to open the URL
 29 
 30     time.sleep(maxwait*random.random())
 31     #response = urllib2.urlopen(req)
 32     opener = urllib2.build_opener()
 33     response = opener.open(req).read()

Works fine on one machine (with eclipse and Firefox directly) but fails while running it at the terminal on another machine (different IP) with the following error.

urllib2.HTTPError: HTTP Error 403: Forbidden

Any leads? Thanks guys!