The only effective treatment of a clogged sewer line is a professional plumber with professional equipment. Plunging may clear a clogged toilet, but not the sewer line. Chemicals are not effective either. I will venture a guess that there is something in the line that is catching debris that forms a clog. When drain suddenly cleared, it was because this clog broke loose at least enough to allow the drainage. It may take a camera inspection to find the real cause. The cure will depend on the results of the inspection.