I have a 2007 F150 4x4, with 5.4 V8.... 109K miles.
Just recently, my power steering has become intermittently stiff, as if there the power steering pump is failing. Fluid levels are fine, and clean. The stiffness is not consistent through the turn, kinda' "on" and "off," if that makes any sense.
I would appreciate any insight, as I need to make sure I have a grasp on what is going on... I live in a new area, and seeking out a mechanic (in case I cannot fix the problem myself... anything post late 1990s nukes my mechanic abilities), and have cautious of being taken for a ride.
Check the joints on the steering shaft,the one the steering rack end is the most likely one,common problem with these trucks,ford tech told me this. Cost of the shaft here in Canada is around 400.00 from ford, Napa is around the same thing. Joints are not serviceable you have to replace the shaft. Good luck,hope this helps.
Have you inspected your drive belt and the PS pump pulley? If the pulley is loose on the center bushing, you'll have to replace it and it requires a special puller. This is probably not your problem - just offering a thought because I just had to replace the pulley on mine.