power steering in 1995 crown vic

driving straight its ok, when i go to turn the steering wheel starts to jump, and its hard to turn. the power steering fluid is full, what could be the problem, and what can i do to fix it..thanks.

There is a small u-joint in the steering shaft just below the brake booster. It is probably freezing up. Take the shaft loos from the box and see if the u-joint is stiff. If it is, use some PB Blaster and work it until it frees up.