I have a 05 impala, i just took it in today and had a new fuel pump regulator put in because i was getting horrible gas millage, hopefully that was the problem, anyways, now theres a new problem. My engine shakes while in park or at a stop, and the lights also dim a little while its shaking. it doesnt do it all the time and sometimes its worse than others. Sometimes itl make the whole car shake. Any suggestions? Also i dont have an rpm gauge in my car but when its in park and i rev the engine it only goes so high n will start going up n down. Is there a limiter on it?