This same scenario has happened with three different internet connections. At first each internet connection works fine.
This is from the same machine?
To test Michael's DNS theory, if you don't have the files I mentioned above try putting 22.214.171.124 and/or 126.96.36.199 into your Domain Name Server settings in Network for the interface you're using.
Edit - In Terminal type "sudo crontab -l" (the letter L, and minus the quotes), hit Return, enter your password. It should return "crontab: no crontab for root". Anything else and your DNS settings have been compromised.