shifting soundstage>

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? 

Its the recordings. But if you really want to be certain then get a recording you know this does not happen with and play it. Unless you know one I would get the XLO Test CD. It has a track recorded in an empty room with 1 microphones and Doug Sax talking as he walks around the room telling you where he is in relation to the mic. You can hear his voice, footsteps, and he hits a clavis every now and then for reference. He even walks behind the mic and if everything is set up right its one of the cooler demos out there. Plus you get the demagnetizing tracks which are used all the time. Totally worth it.

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.

It is the recordings. A soundstage of an audio system does not shift back and forth; it is a set of components that operates within strict parameters. Unless you move the speakers, change gear or cables, it is fixed.

If the movement is so pronounced that it is unmistakably not the recordings, then you have a bad component somewhere in the chain.

There is another possibility; you could have something amiss happening with your hearing. 

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?