Im thinking youre actually having a fuel delivery or ignition issue. I think its coincidental that the problem is most prevelant when in overdrive. Maybe its coil related or the igniton module is starting to fail and showing up when its hot.

 

Not if it's the type of shutter I'm thinking he has. Which, if it is, I'd shoot for a trans issue..maybe engine. I've had my car do it before too. Downshift or give it more throttle and it goes away. I've never actually let it happen long enough to see if it went away by itself since it's only happened going up an incline.