I have heard the same thing about carbon build up. Might run some Sea Foam through it to clean out the carbon. I find it at the auto parts stores for less that $6.00 a can. A lot of us motorcycle riders use it with good results. Just a can put it in the tank and use the octane the car calls for.It might take a couple of cans/tanks to clear it out. Or it could be a knock sensor, if the car has one which adjusts the timing when it senses the pre detonation. Another good one recommended by a Volkswagen mechanic it a product made by Chevron called Techroline (sp?). It might be easier to find. Good luck!