Driving Problem for 2003 MPV

Hi, I have a 2003 MPV. Since the date I got my MPV, it has problem. Sometimes I step on the accelerator, the car will accelerate suddenly (like a sudden burst). However, most of time, it will just accelerate smoothly. Does anyone know why the car will accelerate suddenly? I know it happens more during the winter time. I took the car to dealers two times and found no problem. Can anyone e-mail me your answer at chen1111@yahoo.com as I don't read this group regulary? Thanks!