im guessing you have an automatic. your engine will jerk if its cold.. when it is fully warmed up it doenst jerk does it? cause when u start your car.. ur rpm ranges around 1.5 - 2.5k .. something above normal. when you switch it into gears, the gear is then engaged around that rpm.. and goes down instantly which creates the jerk.. its like haven ur car on neutral and pushen on the gas around 2-3k. then dropping it to drive..