There are not many things more annoying to me than drivers who slow down and speed up repeatedly when I am around them on the interstate. When I get on a road that isn't too crowded I like to set the cruise control at about 5-7 mph over the speed limit and just get in the right lane and mosey down the road. I've hit stretches where I haven't even had to touch the brakes for over two hours; this isn't at 2am either, it's at fairly normal times during the day. What really irks me though, is when some moron will come flying by me and hop into the right lane then slow down. I move quickly up behind them and consequently have to pull into the left land and pass them, usually while they are talking on their cell phone or eating or doing something similar that does not require you to slow down. I pass them, move back into the right lane and keep going on my cruise control. Then, once they are done with whatever it is that they were doing they decide to speed back up and tailgate me for a bit as if they're expecting me to speed up just to sit them or are affronted by the fact that I passed them and then got in front of their car. As the car passes me the driver/passenger shoot me a look as if to say "what the f*** are you doing in front of me?"
Of course, not everyone does that. But I remember those that do and hate them.