Regularly get "Server Not Found" then always works with a retry

My default browser is Firefox 3.6.6 and my backup is IE 8.0.6001. I use NIS 16.8.  Recently I started encountering error while trying to access webpages.  Firefox  Regularly get "Server Not Found" then always works with a retry, while IE would say "Internet Explorer cannot display the webpage".  I was wondering if NIS had something to do with this problem?