I've expereinced this problem several times in the last few years autocrossing, on two different '94 LS's.
The only difference for myself, it only occured at very high RPM's, IE 6500 and above, and only with quick back and forth transitions. Car would steer fine one way and then be stiff the other, as in a slalom.
The one thing I found that would help alleviate, and in most situations solve the problem was Redline power steering fluid. Expensive, yes, but the problem rarely shows it's head
Ryan