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.

The stove does not have any way to adjust the top burners. It does NOT have a standing pilot light either.

Since I live in Mexico, is it possible that the LP gas I use is simply not well refined and THAT is the problem ?