When I'm driving on the interstate, I set my.

That is typical of the transmission's torque converter unlocking and re-locking for better fuel mileage. It is supposed to unlock when the throttle is almost fully released, as in preparation for coming to a stop, and at wide-open throttle. Most commonly it will unlock when the cruise control fully relaxes its throttle cable when going downhill and the car is picking up speed. Once the car is on level ground or going uphill, the torque converter will lock up again when the cruise control pulls on the throttle. If the unlocking occurs randomly on level roads, suspect a problem with the throttle position sensor.