Ford COP systems are like that. If there are no codes, slight misfire/surge issues can be difficult to trace. I'm sure the technician was as frustrated as you at not being able to pinpoint the problem. These are OE engineering issues usually, not a lack of effort or competence. I would suggest a throttle body cleaning and a careful mass air flow sensor cleaning. Next step might be a "smoke machine" test to look for intake manifold vacuum
Chino Autotech Inc.