Camera jitter on movement

So, I'm working on a 3D FPS character controller.

My first controller was very jittery/stuttering, and after a bit of research, I've found out that it might have been cause due to having the character movement in FixedUpdate() and the camera movement in Update().

So this time, as many suggested, I've implemented them both in Update() and although the stuttering is reduced and inconsistent, it is still present.

The issue and how to reproduce it: It seems smooth when you move only the character or only the camera. It occurs when you move both the player and the camera at the same time.

This is the repo for the project: link text

Please help me. How can I identify the issue?