adaptive cruise control, so I am just chugging along at 78 mph. This car comes up behind me, passes me, and merges in front of me. It then (and I have no idea why people do this) slowed down to maybe 75, which was the speed limit. So I start gaining, pass him by, and merge in front of him. I see in my rearview mirror that the guy jerks into the left lane and speeds up. As he passes me, he gives me a glare, but then when he merges in front of me, he again slows down, and we repeat the dance again. This time when he passes me he is REALLY mad, but I don't know how to use my hands to say "Dude, I am on cruise control, you are the only one doing anything here".

I was laughing more than anything, because it was so absurd, but you never know who on the road is going to be a crazy, so I reduced my cruise speed by a few mph, and let him get ahead.