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What grade fuel are you using, reg, mid grade or premium?
Are you using the CORRECT plugs for the car, or something else? Should be using just the normal Bosch "Super" plugs, non-resistor plugs. The platinum tipped or split-fire or other fancy plugs don't work as well as the regular ol copper core Bosch plugs.
Also you could pull the plugs to check for deposits, possibly could be valve stem seals, or at 300,000 miles could need valve guides.
Gilly
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