"S/he is in the lane next to me, holding speed. I want to pass, or go behind so I can move towards my exit. As I either speed up or slow down, s/he matches my speed so I can't. I either nail the gas or brake lightly to accomplish my lane change. As soon as I do, so does s/he."

As I said before - "flocking behavior" , which is a subconscious reflex. It's also flocking annoying ...