Switch over to a higher octane 92-94 Top Tier Gasoline, for a few months and see it go away,Chevron,Shell, Texaco,Exxon/Mobil, Sunoco, B.P. any major name Top Tier gas and try 1 bottle of Chevron Techcron or Amsoil P.I. Fuel Treatment, or Lucas Fuel Treatments every 1000-3000 miles, may be cheap...