Why does your car jerk when you pick up speed 15-20 miles but it doesn't do it afterwards?

Erratic acceleration at low speeds could be caused by:

1) faulty fuel injection or a faulty jet on a carb on older model vehicles
2) A cold engine in some wintry climate conditions (or an engine filled with cold, heavy non-multigrade oil)
3) Poor use of a manual gearbox - incorrect selection of gears
4) A faulty accelerator linkage