30 * time_step is the top speed, but I don't think that's the problem.

I just made a new project with the same level and player model, same horizontal movement code, but a simple jump system, and now the issue is gone. Also, when the player glides against the vent, the reaction isn't so jittery. I have other code in the player of course, so I really don't know what's causing this problem. I'm going to do more tests and see if I can fix it.

EDIT: Ok, I replaced my old vertical movement code with the simple jump code and the issue has disappeared. I guess I'll go and try changing certain lines to see what specific code is causing the problem, and see if I can change it.

EDIT 2: Alright, so apparently it's the code that allows the player to go up slopes and stairs easily. More specifically, it's the code that snaps the player to the ground/slope (using my.z = ) so it's a smooth climb up or down. I guess I just need to find a different way to accomplish this.

Thanks to everyone who helped!