I'm having same problem on my '05 F-350 dually. It just started recently, and is, intermittent!! I have checked the front end and everything is ok ,not loose, not too tight.
I flushed out power steering system and put in new fluid, and not any better. Sometimes turning around corner, I have to pull the wheel back myself, it does not return to center like it should. Left or right turns, doesn't matter. Trying to go in a straight line can be difficult at times too. Once again, this is intermittent, and temperature is not an issue. These problems can happen cold or hot.
Any help, greatly appreciated.