I checked distributor, wires and plugs. All in good to great condition. No fluids in cylinders. Upper head section looks fine. VERY well maintained car - garage kept. Replaced in past year or so due to wear and tear- Distributor, spark plugs, fuel filter, fuel pump (in gas tank) and starter relay. Was driving home from work, turned onto interstate and car started sputtering then sounded like it was only running on 3 cylinders. Pulled over, checked it out. Found no clear issues so drove it gingerly home. Any Ideas? Thanks for your input. I'm only owner. Bought new in 1991. Had / maintained this baby for last 21 years.