Same problem here on Lenovo Y500 Windows 8
Tried different solutions, none worked. The problem is prevalent with online games, probably an ftp access as well.
One thing I think no one mentioned - my internet is not working properly on my present wifi, which is free access, without password.
But I tried it at my parents, where I have to connect using password and there was no problem at all.
So I think the problem is some kind of coincidence of Win 8 driver and wifi-router settings. Unfortunately I cannot test it because I do not have direct access to the router now.
BUT, as I am fed up now with this, today I am getting some cheap usb wifi card and disabling this intel's "crap"(sorry but the problem is so common on different laptops, it is all over the place for more than half year, without solution..). I hope there will be no problem kicking it out of system and using new wifi receiver, so I can report tommorow how it goes.