Ford Repair: 93 ford esort, hilly area, fifth gear

Question
i just bought this car, cheap it drives pretty well, but at 60+mph it starts loosing power in fifth gear. if i put it in fourth i can keep it going at 60+. i've put injector cleaner in the tank, what else should i try? sparkplugs, fuel filter, etc.??

Answer
i've been holding off on answering this one until i thought i could give you a good answer and without anymore info that will be hard so,
when it loses power is it running rough or still smooth-is rpm going higher like possibly clutch slipping-do you live in a hilly area-does it lose power on level ground-you could have things like slightly restricted convertor that is not affecting lower gears-yet!!-definitely have plugs and fuel filter checked-don't put to much cleaner in the tank it can actually foul plugs with to much.