2000 Plymouth Neon "Studder"?

I have a Plymouth Neon that has been having some trouble. When we first bought it, it was okay, now it does a "studder" thing. By studder I mean (only going uphill) it drives as if someone were putting their foot on the gas briefly, then releasing quickly over and over again. Like a jerking motion forward like it wants to go smooth but can't. We checked the battery and replaced it (as was needed). Replaced spark plugs, checked transmission(which was fine) now we're thinking maybe the motor mount? It definately needs to be replaced, but would this be the cause of the such big problem? We were also thinking maybe a fuel filter? what would the signs of these two things be?