I have a 98 mustang gt with a stock 4.6 thats running realy ruff . when its idling its idling kinda ruff and the motor seeming to be louder then nomarl and when u drive it it shuts off after about a mile but will start back up . then go alittle bit and do it again . when its at idle if u take the oil cap off it seems to smooth out alittle but still not how it should be any advice would be helpful thanks. by the way theres no motor noises like tapping or knocking