2002 Steering

I have a 2002 Sihouette that has an interesting steering problem. While parking the van or steering at low engine RPM and slow vehicle speed (like in a parking lot), occasionally the effort to turn the steering wheel increases to the point of not being able to steer. Increasing the RPM's does not help. I am inclined to think it is more related to forward or backward motion of the vehicle. Has anyone encountered this before? There is never a problem with normal road driving and turning corners. I have checked the power steering fluid level, inspected the belt, and inspected the steering and linkage for binding. Thanks in advance.