I've seen several users stating that lately, I am a very frequent user of this site myself and have not seen this at all, so it is not the site uptime per se. I have tried lots of different security setting with my IE7 browser trying to find if there could be some setting making a block but didn't find anything. It has been suggested that some LAN firewall (for example corporate, school, university ?) could be blocking sites, but it has not been confirmed.