My g/f's car is a 1997 Plymouth Breeze and keeps dying trying to go up any incline??

She's been noticing it more and more that it has horrible gas milage, can't go up any incline with any acceleration. Rare but sometimes going up a hill it makes a thump trying to go up a hill too. I was looking at it last night and if you floor it, its like its trying to stall out. Almost like a rev then die rev then die sound. Also if its in park its fine but as soon as it goes to drive it starts to make a dying noise but it never does. could anyone explain it to me and not give me a queer answer like "take it and throw it away. lol" seriously i need to know whats wrong