Thanks again for the suggestions.
I think that some careful dry-firing is in order to see if I'm moving the sight left as I break the shot.
I don't think it's a stance problem because shooting from a rest doesn't change the problem. Might just be how I'm lining up the sights....or not quite lining them up.
Could be an "old eyes" issue, too. With the current state of my vision and corrective lenses, either the target can be clear and both sights blurry, or the front sight can be clear, the rear so-so and the target completely gone. About the best I can do is to get the pistol pointed at the bull using the "distance" part of the glasses, then try to not move the gun as I shift my vision to the close-up part of the lens.
I don't expect to become a tack-driver, but I would like to develop enough skill to let me enjoy an occasional IDPA or similar match.