It could be DNS failure. Lately I've experienced an intermittant problem with
www.yahoo.com not being accessible. Other subdomains in Yahoo like groups.yahoo.com are still available, just not
www.yahoo.com -- then when I try again a hour or two later it works again. Remember the news stories about 'pharming' (DNS poisoning) being the next step in online crime scams? It's possible that attempts at this are causing problems.