This is a reposting with new (less) information. The minivan is hesitating when you drive. As you drive on the highway, it feels like you are moving your foot on and off of the accelerator. There is no check engine light and the mechanics can’t replicate it or figure out what could cause it.
One of these MAY or MAY NOT be your culprit:
Faulty ignition system
Clogged or dirty fuel injectors
Low fuel pressure (weak fuel pump, dirty fuel filter or damaged fuel lines)
Faulty emissions or engine control system