We have double sinks in our master bath. Every now and then the one on the right has the drain slow down. I have to take apart the drain from the stopper in the bowl, to the pipes under the sink, to the connection to the drain line into the wall and for lack of other description, a nasty black film that is on the inside of the pipes, up to 1/4 inch thick in spots. The large chunks come off fairly easy, but it is hard to completely remove the material. It is almost a rubbery, slimy substance. First, what is it? Second, why does it only happen in one sink in the same bathroom (In the 8 yrs we've been in the house)? Third, how do I get rid of it?