pjl

Looking for some advice. '05 F350 6.0 c/c dually 4x4 150 000 kms. The power steering intermittently pulls in either direction. I have had the front jacked up and could find no faults in the front end, tight and smooth lock to lock. Steering gear feels tight, no slop. Sometimes if turning at an intersection, the steering won't self center, manually have to straighten wheel, then hold wheel against hydraulic assist pressure to travel straight, or over compensate and the steering will pull in other direction. Also had it where travelling down the road, assist slowly goes from side to side forcing the truck to wander, not twitchy like a castor problem but as if the hydraulic assist won't center itself. Most of the time it drives and steers as new. Tried flushing power steering system for contaminants. I need to confirm the problem before buying parts. Dealer suggested checking front u-joints, but I can't see how it would pull in either direction intermittently. I originally thought of seizing balljoints, but it comes and goes and isn't difficult to turn steering wheel when it does it. I've never had a power steering box apart to find how it senses and applies power assist, can't find a parts break down for the box either. My experience is in aircraft maintenance, so I'm not oblivious to the potential safety issues, but I don't want to throw rather expensive parts at it either. Thanks for any constructive advice.