This has been covered many times. Fords cruise control is very lazy at trying to maintain a set speed. My 02 X is the same way. I have to watch ahead for any type of incline in the road and help the cruise along when at the base of the hill. the set speed will drop sometimes several mph before the cruise tries to increase speed, by then it`s too late and the engine needs to downshift to regain that speed. it is frustrating, staying on top of it in hilly areas is the only way to go. a pain because you have to do what the cruise should be programmed to do.
BTW, are you running with the Tow Haul engaged?