Although I did suggest to the mechanic to check the plenum, the problem does seem to be fixed. They cleaned the fuel injection system and did an air induction (or reduction?) and the car is running much smoother. There is no hesitation after 45 mph and the car accelerates beautifully. The car has over 171,000 miles, so I know I'm not getting the greatest of gas mileage. I'd like to improve that. What are some simple things I can change to help imrove fuel consumption? The following is a list of what already has been replaced:
Spark Plugs, wires, distributor cap, rotor, map sensor, O2 sensor, air filter, catalytic converter, and the mechanic added some kind of special chemical to keep my fuel injection clean.