Hello, I have a 2005 1.2 8v Natural Power Punto, it's used and has some problems, the main one being that after 10 minutes of driving the car idle becomes rough (noticed on traffic lights). The whole car vibrates every few seconds and if I press the throttle, the revs drop from 900 to anywhere between 700-400. The whole car vibrates violently when this happens, the lights dim and when the revs rise it goes back to normal. I changes the spark plug vires yesterday because of something unrelated and it seemed to be fixed, but today it happened again so I guess it wasn't a 2 birds 1 stone kind of deal. Any help is appretiated.