I wouldn't be surprised if some of them are down for good...
One problem with Installer is that its so open, and we have too many random kids wanting to start their own repository, and then letting them die after losing interest in a few weeks. So things come up and then disappear after some time. But with Installer (at least with 3.0 and earlier), they don't just disappear, they lag your Installer until it is removed from your listing. Which is why repo source can help you speed things up -- we don't go down unless our host is down

I am not sure if that will change in Installer 4; but I know for sure Installer 4 sources layout is different, so I'll need to find some time next week to update our system here, and see how that goes, I guess.