shifting soundstage>

the speakers are in phase.

Some discs do that on purpose. Hendrix use to love bouncing between channels. The more likely explanation is that you have a very narrow  sweet spot and you are moving your head. That will make instruments wander. For the perfect image you have to be in perfect phase between the speakers. As you change phase by drifting to one side or the other the instruments will appear to move. It is just the nature of the beast. 

that makes perfect sense; thanks.
does the "sweet spot" change with speaker placement?

Is it only on specific recordings and at specific moments in those recordings?  If so, then it's the recordings.  If it's somewhat random, then it could be equipment, but it could also be you.  If you listen for problems, then you're more likely to hear problems.

Ironically, it is more apparent on the most detailed recordings with the most specific and dimensional soundstage. On recordings with less focus it is not as noticeable, which makes sense. As mentioned, the placement shifts are subtle and noticeable only with listening concentration, so yes, it may be the result of critical listening.  In other words, yes, it could be me. 
The room is very large with an extremely high ceiling without acoustical treatment, which I suspect may play a role as well.