I've tried to make a log of when this happens but nothing seems to really correlate. At times it feels like my car is "chugging" as if it is trying to get into the next gear but "chugs" a little bit. It happens at all different speeds but doesn't happen all of the time. I am questioning this because my sister has the same car (mine is a 4 cyl hers is a 6 cyl). Every time it happens it does it for like 20 seconds then goes away. We use the same dealership for the maintenance of our cars. The dealer always tells us they don't know what is wrong because it doesn't happen when they test drive it - but that is because it doesn't happen all of the time and of course it doesn't happen when they take it out. Either way, they recently told my sister it was her fuel line (that it must have been punctured by mice) so they replaced it. As of yet, she hasn't had the problem. My car though is still doing it... and my fuel line is fine. What is really happening here??