Its been happening for a month or so and I figured it was down to my crummy wifi, but it even happens over my 3g connection and on a friends wifi.
It does seem completely random as to when it happens, but its usually when opening a thread or doing a search.
Just as I went to post this message, the tinymce textarea didn't appear! Hit refresh and got the 'Server not found' error. Hit refresh again, and the page loaded.
My guess is a routing issue as the server/site is lightning fast at all other times.