Found spark plug gap closed. Replaced plug and it did it again. Kept opening gap every time this happened and one day it stopped. That was 25000 mi. ago now it's doing it again. Mechanic says it's a piece of carbon on top the piston and who knows, he was not real sure but I've never heard of such a thing till I got on this website and found out I'm not alone. There must be a problem with the 2.5. I wish I'd never bought it. Even with top of the line extended warranty like I have forget Nissan owning up to a problem, they say it's normal maintenance, even thought it's major engine work to get to the top of the piston and see what's going on.Figure that one out!