who can fix sagging brownstone stairs?

I’m helping the owners (a religious institution) of a double brownstone in Boerum Hill find someone to repair a sagging interior run of steps. It’s your typical brownstone stairrcase, with winders at each end. The entire chassis of the stair moves up and down an inch, and individual treads are disengaged from the stringer. It isn’t near collapse but who knows? It’d be a great help to find someone wirh experience in this sort of thing, with references, who could do this affordably and well. thanks!