Sorry, but cruise control cannot overcome gravity when gravity is stronger than wind drag. At that speed the torque converter is not locked so you do not get any noticeable engine braking. Also, cruies control is not really effective at speeds under 30 mph.
You might be able to prove what Bill is saying by turning off your OD. Then the engine braking might be able to keep it under 25 but if you fall below that the cruise control would apply gas to maintain the set speed.
I had a 97 Ford Ranger. It was a problem with the actual Arm (In the steering column). My problem also included the windshield washer pump. I believe it was a pretty common problem.
Part of the "problem" is that as the speed increases, the transmission up shifts. Putting it in first will help, but, as Bill-E points-out, gravity will win.
