The jib car position controls the amount of twist in the sail. When the wind picks up the jib stretches and the top twists open. If the top is too open the jib may flutter and you won't be able to point well. To close the top of jib you need to pull the clew down (towards the center of the earth). To pull the clew down move the car forward. When the wind eases the jib stretches less and the top of the jib will be closed too much. To open the top of jib you need to pull the clew down less, therefore, move the jib car aft. If the top is too closed you will be slow.
If the leech is stretched as Nolatom suggested, you may be able to to reduce the fluttering by tightened a leech control line.