Re: High Left
POI shifts that come and go can be a cheek weld/cheek pressure/parallax issue. Be sure they are correct and consistent.
Another issue can be 'dragging wood' where finger contact alongside the stock during trigger pull can affect/shift the POI.
Try and make sure that the finger only contacts the rifle at and behind the knuckle, and that the trigger is riding centered on the first finger pad, with no contact against any part of the rifle between trigger and knuckle.
The web of the thumb should be consistently centered in the hand grip area.
Butt positon in the shoulder should be identical and shoulder pressure must be consistent.
There can be a slight chance that the scope's axis is not centered above to the bore line, or that the reticle is not properly levelled; and horizontal POI shifts at different distances can be symptoms of such conditions. This is an expected issue with side-offset scopes, like the side-shifted scope on Garand Sniper models.
It may not seem realistic that some of these things can be significant, but they are.
Greg