I have to guess that you have an automatic transmission. If so, the most likely suspect would be the torque converter is unlocking inappropriately. It locks up in the higher gears above a minimum speed for better fuel mileage, and when it does, the engine speed will drop by about 200 rpm. It should not unlock when you're cruising at a steady speed on the highway, but things that WILL make it unlock include the vehicle drops below that minimum speed, (typically around 40 - 45 mph), tapping the brake pedal, as in preparation for slowing down or stopping, same with releasing the accelerator pedal, or pressing the pedal beyond a certain distance to the floor. Those last two could be caused by the cruise control if the car is going downhill or uphill. A slightly misadjusted brake light switch can make the torque converter kick out momentarily. Often the "cancel" signal from the brake pedal is on a different circuit in the brake light switch so the cruise control may not disengage at the same time.

Many cars in the last 15 years will automatically downshift to slow down if the speed can't be reduced by the cruise control alone. That happens when going down steep hills, and it can be very disconcerting if you've never experienced it before.

If this starts to occur on a more regular basis, including when you're not using the cruise control, try holding the brake pedal up with your toes. If the problem stops, suspect the brake light switch or its adjustment. To verify if this is what's happening and to see what it feels like, drive at a steady highway speed, hold the accelerator perfectly steady, and momentarily tap the brake pedal with your left foot. You should hear the engine speed increase, then go back down a few seconds later. You'll see that on the tachometer too if you have one.