Stairs There is a bug ! (Psi-Ops: The Mindgate Conspiracy)

So decided to go in the game, Psi-Ops - The Mindgate Conspiracy on the PC, everything is fine played myself, and you ! When the stairs leading down, or up, the person falls off ladder and dies :( If anyone has passed this game, what's going on what's wrong in it, I played playstation 2 for a long time here miss the old fashioned way.