I don't think this would work but it's the only thing that came up in my tired mind
Start > Run, ipconfig /flushdns
and see if the sites work
Hello sir, to assist you first I will need you to restart your computer. Click the circular button on the bottom left of your screen, then click 'Shut Down', count to 500 and then turn your computer back on.
What operating system are you running? The first thing that comes to mind is clearing your DNS cache, maybe trying OpenDNS just to see if that helps.. I've had similar problems before and it just took a DNS flush & reboot to fix.
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I was just making fun of tech support
What happens when you ping the subdomains that aren't working? Try on the computer you're having problems with and one that's working.
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What ISP are you using? Some overload those servers with shared hosting so badly nothing works well.