What is wrong with my power steering?

I'm having some issues with my power steering. The fluid level is good, and the belt looks okay, however I am aware that it is probably quite old. I have a 1995 Camry, and sometimes on wet/rainy days, if I were to turn the wheel all the way in one direction, the belt will slip and screech, and the steering wheel may get thrown in the opposite direction. This has never been a major issue, but now I noticed that while driving on a long road trip, my steering would go in and out... most of the time it was normal, but occasionally it would either pull, or the steering wheel would bump left and right (almost as if my tires weren't balanced, but with less of a pattern). I am wondering if a slipping belt could cause this problem. The belt for the PS is separate from the other accessories. In addition, the pulley on the PS pump has quite a bit of wobble. Could this just be a loose belt, or might I have some other issues... How should I go about repairing this?