So, finally found the problem: gas line gets totally squished up against the edge of the hole in the floor under the sink when the slider goes out. Looks like some things shifted over time, and the other hoses and electrical etc., pushed the one gas line, which just happened to be for the furnace, up against the edge of the hole and compressed it enough to prevent function. Going to work on enlarging the hole today. Had to take out not only the back panel under the sink cupboard, but the flooring of the cupboard as well to get to it, in case anyone else needs to access this area due to compressed things.