My Sentra did this to me yesterday really really bad. I accidentally staulled it, and started it and got going, when I was in traffic in 3rd gear going around 2500rpms the car started BUCKING bad, coughing, I was not letting off the gas peddle but it was loosing power and actually slowing down about 2-3 mph and going again then slowing down, son of a ***** this thing was pissing me off. It kept on for some reason in 3rd. I tried 2nd, no problems, and ever since I haven't had a problem again. Uggg I don't get it..... But I have noticed it do it not to that extreme but on other ocasions...