Why would a 1998 Pontiac Sunfire jerk when driving?

Have you checked your spark plugs and the leads coming from your ignition coil? If you replace them, do it one lead at a time so you dont mess up the firing order. Get your buddy to drop the wire down to you. Get a good safe jack. Its freaky down there.

If you find that the jerk is felt in the steering wheel, (the wheel is actually jerking in your hand), you may well have a broken strut mount. If this is the case, it can be dangerous and needs to be addressed .