1991 Isuzu Stylus slow?

I have a 91 isuzu stlus that I just traded my dirt bike for.
first off it gets only 20 miles to the gallon. I have replaced the spark plugs, wires, cap, rotor, air filter, and oil filter. It still gets terrible gas mileage. What should I do about that?

secondly it idles rough and I have taken it to a mechanic and he did a diagnostic and he said it passed with flying colors. It has a new muffler, water pump and radiator.
what about all that?

and lastly, when I am driving sometimes the acceleration dies, it just looses speed and then picks back up, and this happens randomly a have not been able to accociate a consistent pattern, could this be why the car is getting such bad gas mileage? Its inly a 1.6 I would assume that it should get around 30+ mpg.
thanks
Chris