I've had the problem many times from the instructor's point of view - Eventually the maintainance organisation will twig to it. If the shimmy damper is not worn and all oil levels and air pressures are correct, the oleo has to be re shimmed to make it stiffer to turn. This crude method works a treat and has no effect on the ground nosewheel steering. The problem is frightening however as when it gets really bad, no amount of up elevator during the landing roll will cure it.