If it works sometimes but doesn't work other times, it's probably the fault of your ISP. It's possible that they have two different types of dial-in servers, and that each type uses different authentication methods. The next time you have a problem connecting, view the properties for your dial-up connection. Select the Security tab, and check the 'Show terminal window' checkbox. This will allow you to watch the conversation between your computer and the server. Watching it may make the problem apparent. Perhaps your computer doesn't recognize the username prompt, or is issuing the wrong command to start the PPP session.
This was first published in June 2001