I think I know the problem. Not 100% sure, since this is a little weird. But I have a theory.
The 500 steering box is ... ummm ... an ancient artifact often referred as operating on a level of mammal feces. It has three big issues.
1. The soft metal the gears are made to wear quickly. But I don't think this is the case here.
2. The clearances in the box are less than precise.
3. The box doesn't like a lot of power steering fluid volume.
Your pump flows a lot more volume than the box was designed for. Combine that with the looser clearances ... and I am pretty sure it will create a "wandering condition."
Unfortunately, I don't know a simple way to test my theory ... other than replacing the steering box.
If you decide to not live with it ... and buy a box ... ideally, I suggest you use the new Delphi designed/Saginaw manufactured 670 Series ... blueprinted by a shop that knows their poop. Lot more money. Lot better box. $775 from Turn One Steering: http://www.turnone-steering.com/#!on...egory=12405583