shifting soundstage>

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.