Other thing would be to watch the TB and APP (gas pedal) at the same time on a live data scanner. That way you can make sure the gas pedal is not moving or doing things on it's own when you have the cruise set.
I don't know for certain if it works this way but the APP should be the same but because the computer controls the TB, that should be the only one that increases and decreases with uphills and downhills as it needs or doesnt need as much throttle input.
I've never run into this issue with a vehicle at work/home before but I would maybe look there as an idea.
