What do you mean by "always down after 15minutes"? That sounds more like a server connectivity issue. First, you should investigate whether the problem you're facing is just the network or the entire server keeps rebooting. Secondly, if you're the admin, haven't you try reinstalling the OS? Even if reinstalling the OS doesn't fix it, then it's definitely something that cannot be fixed in your control, you need the help of your host. They should fix it for you. So please, take the first step I suggested.