Car Shakes during IDLE

[praivo]

I had this problem too (rough idle with lights on, OK without). Then I filled up with "premium" 98-octane fuel once, just out of curiosity, and the problem was gone within 30 seconds after re-starting the car! When I was waiting at an intersection to exit the station, the idle was already completely smooth. And it really *was* because of the fuel because the stuff I used to buy before makes the problem reappear - again, almost instantly.