1998 Subaru Forester

Engine Performance problem
1998 Subaru Forester 4 cyl All Wheel Drive Manual 170, 00 miles

This was all done by a mechanic. Had a code to replace cats, did that. Was told I needed a tune up, did that, New plugs, wires fuel filter air filter. Still my engine was missing. We have now replaced the mass air sensor.I am getting no code and I still have a slight miss. Why?
July 4, 2010.

Engine running rough or misfiring can be cause by the following: 1. Defective spark plug
2. Inadequate spark/coil, defective spark plug wire.
3. Lack of compression
4. False air leakage.
5. Faulty fuel injectors.
6. Insufficient fuel pressure.
7. Contaminated fuel.
8. EGR valve that is leaking.
9. Oxygen sensors.
10. Throttle position sensor.
11. Idle air control valve.

Jul 4, 2010.