Power loss

Hi, it runs and drives very nice runs smooth, but is lacking power somewhere. When I start to go up anything that puts a load on the truck such as a hill it just acts like it has no more power and just keeps slowing down until I crest a hill and seems to be fine if I am on flat ground or down hill. I know it is kind of vague but that is the only way I can explain it with out having someone ride in it. Any input would be great. thanks