NavList:

The psychovisual accommodation remarks from the other replies are certainly worth looking into, but I'm wondering whether your mirror might be slightly out of flat.  If you're using an AH mirror in the usual way, with handheld sextant at a comfortable constant height above ground, then when approaching or receding from the AH, you're also observing celestial bodies at different elevations.  If your mirror is out of flat, the changing ray angle at the AH gets translated to astigmatism, and that (slightly?) astigmatic output beam might be causing you to want to refocus.

Try some flatness tests.  In the home workshop the easiest way is probably an optical flat and a monochromatic light source.  Or, actually, a point source observed at grazing incidence is a pretty good way to assess overall flatness too.  The telescope-making books go into detail about this.

Mirrors can be out of flat from thermal distortion or from simple mechanical loading if they are thin.  For moderate-sized astronomical mirrors the suggested aspect ratio is something like 1:8, but an artificial horizon probably need not be quite that stiff.  What are the dimensions of your mirror, and how is it supported?

Cheers,

Peter