It is important to have functioning fuel injectors. A person can unclog their fuel injectors by buying fuel injector cleaner and putting it in their gas tank every time it almost empty.
There are many little steps when removing the injectors, I would recomend a haynes manual. It's not hard to do, but better if you have the pics in the book to go by.
The primary cause for jerking on a 99 Mercury Cougar is the engine not getting enough fuel. This can be a sign of a clogged fuel filter, bad fuel pump, or clogged fuel injectors.
Rough idle on a 99 Dodge Stratus is often caused by a clogged fuel filter or injectors. This prevents the engine from receiving enough fuel when needed.
I know of two. One is a fine material covering the fuel pick up in the fuel tank. And the other is a cylinder located inside the drivers side chassis rail. From there the fuel goes straight to the fuel injectors.
'99 beetles are fuel injected. Problems can be from clogged injectors or filter, faulty relay, or fuel pump. I am experincing the same problem on my '99 and have it narrowed down to, in my case, either the relay or the pump.