Is the thrust adjustment too tight on the box? There should be a little backlash in the sector shaft when the box is at the center of it's travel. Note, the box itself needs to be centered, not the wheel or any of the linkage.

If the box is too tight (preloaded) it will want to force the sector shaft away from center.

Drop the center link, cycle the box lock to lock and find center. Check for backlash.

I've seen too many people try to overadjust a steering box in an effort to fix a different problem.