Power steering jerking back and forth violently

[surnufz]

What would cause a power steering box to jerk back and forth? I have a sbc with a stock type power steering pump. I also have hydro boost brakes. I'm pretty sure I have plumbed it correctly. I had just started a fresh engine and had the front wheels off the floor and was going to turn the steering wheel back and forth, and as soon as I touched the wheel, it jerked back and forth very violently. I've never seen or heard of this. Can anybody tell me what is wrong? thanks