I ran into this once and solved it by cleaning the filter in the PS tank.

Sometimes these get tossed out by a previous owner. Fastlane will have these and they should have a photo of one so you can see if yours' is still there.

Only one thing can cause foam in a hydraulic system and that is air. If your system sucked air when your fluid is low you will have to bleed the system and then maybe bleed it again.

Try running the car without the fiber PS filter and see if there is any change. A dirty filter can also cause this problem as can a faulty gasket on the PS steering tank top.