Power steering frustration

That behavior indicates air entrainment. What you are feeling is the air bubbles scooting on by. Once the air comes out. it will settle down.
In the picture (post 8) you can see the left and right turn power-chambers, where the air collects. it will come out as you drive.
Do not hold the steering all the way over on the stops, which heats up the oil.
Do not overfill the pump, as when the air gets hot it will expand and puke the oil over the top, making a mess. Just put in enough oil to cover the pump at the bottom of the filler tube, then keep adding oil every few miles to keep the level about there, as the air comes out. After all the air is out you can top it up.... HOT. If you top it up cold, it will puke oil after it heats up. Run at the bottom until it stabilizes.