Hi Donna,
     After reading your post about the spark plugs, I must concur with Mike Thompson's assessment.  Before I entered the ministry, I was a Shop Foreman/Service Manager at a couple of very large GM auto dealerships for over ten years, and am also an ASE Certified Master Technician.  I have seen many "high mileage theories" come and go.  There may be something to this theory, but don't hold your breath, and especially, hold onto your money!
                                               In Love,
                                               Blake Hayes
                                               Pastor - Riley's Creek Baptist Church