Mine did that a few years ago when I took a 150 mile interstate trip at 75 MPH, I never worried about it just figured it was because it had a chance to open up and run like it's made to do. It's used to short trips and stop and go driving, eventually it idled down to where it was.
Same thing here, that is the first time its been under load for an extended period.
Usually the kid just drives around town.
I guess Ill let him drive it around like that a few days and see what it does.