Intermittently cannot login to Facebook

I can usually login to Facebook. Sometimes I cannot login for hours. I have tried using Firefox and Internet Explorer, both eventually time out with "page cannot be displayed." On a friend's phone I can login during this time. On very rare occasions I will be using Facebook when it drops out. Clearing my cache or restarting the computer does not help. This happens a few times per week. What is going on / what can I do???