I have a 2004 Porsche Cayenne S, recently the engine feels like it's skipping when I'm driving between the speeds of 25-50 mph. When I'm in low gears, driving less than 20 mph, it's fine, and when I'm cruising in high gears, it's also fine. It's mostly when I'm accelerating at mid level speeds. Additionally, my gas mileage in the city has been only 9-10 mpg. Is this a faulty spark plug? I just bought the car a few weeks ago, so I don't know if that gas mileage is poorer than usual. I had the car at the Porsche dealership about 3 weeks ago for a PPI full inspection and they said everythig looked good at the time. Also, last time I fueled up, I did not use premium gas, I don't know if that is relevant or not. Please shoot me any insight you have, I would like to hear a mechanics Mercedes star C3 - thanks.