Set the collision box for your staircase to be a flat slope. I’m assuming the collision is a box at each step in the staircase, so what’s happening is your player steps on the first step, then the next, then the next, etc., jumping up (or down) every step, making it look jittery.
So, instead of having the collision set at every step, just make a flat “/” shape collision from the bottom step to the top. That way, the player is walking up a continuous slope, and not jumping up every step.
I hope that made sense and helps the issue!