Is there brown muddy looking shizz inside your radiator / underneath the radiator cap? If so, take it out and clean it out with a hose. If that isn't the case / doesn't do anything, take your water pump off and have a look inside, if it looks foobarred then replace it. If that doesn't do anything. take it to a radiator place and get them to check it out. From what you've said it sounds like a water pump failure to me.