shifting soundstage>

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.

thanks for the test CD suggestion. the recording is certainly a leading suspect which is why I am wondering if many others experience the same malady with high quality recordings. If it is related to the recordings, I would expect that many are experiencing the same observation.

Movement is slight and subtle, but the soundstage is normally very focused and dimensional making the shifts noticeable to me.
Regarding my hearing, a valid question since I am 80 years old. But, if I can hear those subtle movements, how bad can it be?

the speakers are in phase.

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

Thanks to all for this very interesting and enlightening discussion which has offered much credible and helpful explanation.I now have additional information to conger while I am listening.

thanks for your thoughtful responses. 1. no tubes in the signal-path; all SS high-end equipment.
2. Room acoustics were a suspect, but as you alluded, nothing in the room changes when the soundstage stability changes. It seems to occur more with instruments than voice. Is that a clue?