It'll probably come back to worn components in the end, but my understanding is that if the adjustment screw on the steering box is too tight it'll reduce self-centering.

But...people adjust the screw to compensate for wear, sometimes in the steering box itself (i.e. too "loose" at center due to wear in the Pitman gear) and sometimes in other components.

A thorough inspection of the steering components is in order. If they all look good, you might consider loosening the adjustment screw to assist with self-centering. It may make your steering sloppier than you like.

I'm assuming you've already looked at the basics, like fluid level, P/S pump belt condition, fluid condition, etc.