Diana and I made the connection that we're bothered by the way some people drive. We call them thrusters. They don't keep a steady speed, but rather "thrust" and coast over and over again, making for a lot of motion that becomes somewhat uncomfortable over time. Their speed drops down a little, and they give the gas a good push to get it back up. I can only imagine how terrible their gas mileage must be.
They drive you nuts when you're in the car with them, but you can see spot them on the road too. Usually it's when they think they're gonna pass you, so they're next to you, fall behind, then push ahead suddenly, repeat.