1999 Subaru Legacy

Engine Performance problem
1999 Subaru Legacy 4 cyl All Wheel Drive Automatic 170500 miles

The check engine light keeps coming on. (Legacy Outback) The code is PO 303 & 304. The mechanic replaced the coil pack. The light came back on. The spark plugs were replaaced. The light came back on. The mechanic thinks a valve adjustment may be the next thing to do. He says the car runs very smooth and according to the ck engine code it should be running on 2 or 3 cylinders. And running rough. Anyone have any help on this strange problem? Should I continue with this exploratory work and get the valves adjusted?
Bob Wilkinson
June 8, 2009.

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