98 Silverado Power Steering Jerk

[jmnnn]

I have a 98 Silverado 4x4 Ext Cab. While at highway speed, the steering is slightly stiff either side of center, when making gradual turn and appying turing pressure the wheel will suddenly 'give' and the extra force applied makes the wheel jerk a couple inches. This is enough to cause a slight swerve as you are driving. Not so noticable at slower speeds. P/S fluid is OK, pump makes no unusual noises. Only does this on either side of center.