I use the cruize in my GT all the time on the highway. I set it at about 75 and go... uphill/downhill never noticed this symptom. At this speed the rpms are in the sweet spot for power and she flys up the hills no problem and coasts down the other side with no ill effects.
Does it happen as the car is exceeding the set speed due to steep grade? Or just trying to maintain the set speed?
As eamiller says, the power will surge on if you hold the accel stick or tap it rapidly.