I recently started experiencing this very interesting issue. At random, a webpage that might have worked 30 minutes before, would suddenly fail to load, giving me a server nonresponsive. I know this is not true because using a proxy, the web page loads fine. Using the CL to ping the webpage causes a timeout. These intermittent failures last for 10-60 minutes then go away by themselves.
Recently however, en.wikipedia.org has not worked for over 24hrs which leads me to suspect something wrong with my computer's configurations, so I disabled all AV/FW programs but to no avail. My connection is provided by the IT dept of my organization. The only nonstandard setup I have is a switch. Other people using the same connection do not experience this issue. Unless there is a virus, I am baffled.