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.
Give an answer, they learn for the day; suggest a search, they learn for a lifetime.
2007 F150 STX Supercab (4.6 2V V8)