We had the same problem, but it happened on every step. It turned out after having it bolted to the wall for a time, we eventually had to put in a new stairs. It also came to light when the old stairs was removed, that the builder had installed the stairs the wrong way around, so it was pulling away every time it was walked on. We think that the carpet held it together. This stairs had held up for over 30 years. Can you see the back of your stairs, it may be the same thing.