We have recurring clog in our bathroom sink. our house is about 90 years old - we have snaked, drano, etc. The snake goes about 50 feet and finds nothing. The clog starts after only about 20-30 seconds of running water, so it can't be that far down the line. We have run the snake from the wall and know that the pipe doesn't have major obstruction. any ideas? maybe soft residue buildup - anyway to get rid of this if this is the problem? thanks