stumble on hills

92, olds achieva 2.3 or 2.4 (my sister inlaws car) runs fine down hill (dont the all) runs fine on level road but when you come to a hill it wants to stumble almost like a miss or running out of fuel has good fuel pressure plugs are only a couple of months old has new fuel filter. when it starts to stumble on a hill if you hit the gas it kicks down and runs fine. and advise would be greatly apreciated. im into dodges not much of a gm guy