shifting soundstage>

Any tubes in the signal path?

Recording is an art not a science. Someone will probably say this shifting is due to your particular room acoustics, and so you need to hire a consultant and buy a lot of panels and stuff. But if it was the room you would be saying this happens a lot. Instead of "occasionally". If its the room or your setup then it will happen not with any one particular instrument but with anything within a similar frequency range. This I am pretty sure is not the case or you would have said so. 

Not at all uncommon. Especially not with multi-track studio recordings. Can't think of anything like this happening with small ensemble or quartet or solo type classical, but does happen all the time with pop, rock, etc. So the simplest explanation is it sounds the way it sounds because that's how they want it to sound.

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?

Try reversing the speaker leads on one speaker and see if that corrects the problem!

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.

does the "sweet spot" change with speaker placement?

The sweet spot is more a plane than a spot. Technically its a circular section of a vertical plane, whatever the term is for that. There's a couple different things going on.

In terms of pure imaging its where you are equidistant to the speakers. You can toe the speakers in or out, or you can move your head forward or back, or up and down (that's why its a plane not a spot) and as long as you maintain equidistance there will be imaging. Move even slightly left or right and imaging suffers. At first it shifts towards the side you moved closer to but still sounds solid. Move further though and eventually you hear the sound coming from the speaker closest to you instead of magically floating in the room.

But its not only imaging changing. There's also frequency response from the speakers. There's usually a fairly narrow dispersion area within which the speaker sounds nice and smooth. Move a little left or right, up or down, this shifts the tone or balance. With some speakers hardly at all, with others quite a lot. This frequency response shift will of course have some influence on imaging, affecting sounds falling within the affected frequency range. This could cause certain instruments playing certain notes to seem to drift or move.

That's just the direct sound coming right off the speaker. A lot of what you hear includes sounds reflected off all the various surfaces in the room. The size, shape, composition and location of each of these contributes to the total sound. That is why they make dispersion panels, lots of odd shapes designed to break up and reflect sound uniformly with regard to frequency. That is why they make absorption panels designed to absorb certain frequencies.

A typical room has all kinds of stuff going on to the point it seems pretty random, and it sort of is. But listen real close you can start to notice things, and that could well be what is going on here. I still think its almost all recording, but all these other factors are in play as well.

Just to give you some idea what I'm talking about, my room was at one time pretty much empty of everything but the system and a chair. I had a friend over and left a couple albums out leaned against a wall as my pathetic attempt at decorating. He said wow awesome imaging but the left side not quite as solid as the right. WTF?!?!? Never heard that before. But sure enough he was right. How had I missed that?? He leaves and its bugging me until, wait a minute- the album! Put the album away. Rock solid. That one stupid square foot of flat surface was reflecting just enough sound to mess with imaging. So these things really do matter.

Now the way to tell what is what, if the shifting locations maintain their integrity as they move then they are legit and on the recording. Carlos Santana as his guitar moves hard left to hard right sounds crazy crystal clear every bit of the way. But if you hear something rock solid in one spot and then as they play it shifts and at the same time becomes a bit more diffuse and hard to localize, that most likely is the room.

pfrank, yes. Putting the speakers closer together will give you a slightly wider sweet spot as will moving the listening position farther away from the speakers. If a system does not have a sweet spot it is actually defective in some way. The more noticeable the sweet spot the better the system is. IMHE speakers that are more directional like horns and ESLs have a more distinct sweet spot. The problem then becomes that the high frequencies roll of dramatically as you move off center. Ideally the tonal balance should remain the same over the entire listening area. But, the image will only be perfect on the perpendicular line centered between the speakers. So if I want to check out the image and balance while someone is in my listening position I will stand just behind. 

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.

It can be changes in how the listener focuses while listening alone that results in shifts within the soundstage.