I sometimes have a problem accessing sites from my MacBook Air going through Wi-Fi and a Telecom Technicolor VDSL modem-router.
Could it be something to do with DNS settings? I see there's a problem with Windows 8.1 and wonder if it is more widespread. See this thread: http://www.geekzone.co.nz/forums.asp?forumid=39&topicid=129101
Is there a simple way to check this from my Mac?