ghost

here's a question that has been floating around my head for some time now. You now how your car drives when you drive it off the dealer, it runs fast right. I have taken my car to the dealer three or four times to have replace the O2 sensor, crankshaft sensor and abs sensor for LF wheel, well every time I get my car back it seems to run just like when I purchase it and after a couple of weeks later it gets a little sluggish. Is there something wrong with it or is there a way for me to fine tune it or should I take it in for them to do what ever is it that they do?