Violin Moves All Over the Sound Stage

It could be that the recording has two violin players on it but it could be a setup issue with phasing or speaker placement, the speakers too far apart from one another.

There certainly won't be two violinists in the Beethoven Concerto.
Nonsense like that may turn up in today's gimmick-ridden world but not in this classic performance from 1959.

For those after an historic performance the BBC Music Magazine Vol 20, No 4 features Yehudi Menuhin live at the Edinburgh Festival on the 29 August 1948 with the BBC Scottish Orchestra - played this just the other day !!. Coupled with Dona nobis pacem conducted by the composer Ralph Vaughan Williams, recorded November 1936. This is quite a moving performance. RVW seems to be the "go to"  composer for several conductors/orchestras, with many excellent releases recently.

Since Stern was 81 when he died in 2001, I wonder if the recording is digital, or if you just have a digitized copy.  BUT!  He might be moving around the Microphone.

    Isaac Stern  sometimes thinks he's Ian Anderson 
maybe that's the answer 
  yes