Problem with pots getting "crud" on bottoms

Problem with pots getting "crud" on bottoms I have made the change over from natural gas to LP. I am however still getting “crud” on the bottoms of my stainless steel pots/pans. At first it is merely a thin brownish/black in color. But, if I let it go, it will build up to a thick black super hard crud (carbon ? ) that is very difficult to get off.