At the very least (and cheapest), disconnect the lower pump hose to cooler (all 2004 to 2008's have power steering coolers), and drain. Fill with fresh fluid and perhaps some conditioner (I have no experience with any recommended Power steering conditioners). See if the problem clears. If not, I'd say you have a pump or rack issue.
"Never argue with an idiot; they'll only drag you down to their level, then beat you with experience"
2007 F150 STX Supercab (4.6 2V V8), Edge tuner, factory navigation, Mark VIII electric fan conversion with flex-o-lite controller.