Montana hesitates in 4th gear

pontiac
montana

#1

I have a 2000 Pontiac Montana that sat for 18 months in the driveway (NW Ohio) and recently went back into service after having it’s engine replaced. Since then, when lightly accelerating in 4th gear, the car hesitates repeatedly. Accelerating harder, dropping into third gear, or letting off the acceleration altogether makes it go away. The engine has a lifetime warranty on parts AND labor, so I’m curious if this could be related to the engine itself or an assembly issue where it was connected to another component of the car (transmission for example). Any thoughts?