Every so often my drain stops up on the way out to the city's lines. It's very hard to get a plumber out, so sometimes it'll be weeks before that can happen. So here's what I am having trouble with: The line has a tendency to clear itself. So if i get a plumber, the line runs clean as a whistle. Other times it will somewhat clear but I can see the drainage hitting a roadblock and backing up. Other times it will clear on its own and look as if I've had the plumber come out...smooth flow without slowing or stopping. WTH!!???