Mystique or Mistake

I have a 1998 murcury Mystique GS 2.0 . I have had a number of problems and I have finally found won that I am unsure of. I can floor my car and I cant get over 30mph. Their is not responce on the RPM gadge. It did once come out of it and did fine in third (It an automatic tranny) and went right up to 55mph. For months know when it idles the rpms go up adn down from 800rpm up to 2000rpms witch has led me to think of my oxygen sensors. But witch one would it be. I have had the timing belt , altanator, water pump, multipull hot wires, fuel pump, injectors, and the spark plug all replaced in the past two years. I need to know if its wurth my time and what Is wrong with it?
P.S. the tranny and engine leak