Car jerks violently when you let off the accelerator?

Hello, my father had the engine serviced yesterday because it was leaking. Since today, the car has been jerking more than before when you take your foot off the gas, especially in second and third gear. And the jerking always occurs a second later than you've taken your foot off the gas. Otherwise, the car runs perfectly normally; it responds to the accelerator normally when stationary, etc. What could be causing this? My father said some part had been replaced, which required the gearbox to be removed, but I can't remember which.