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.
Violin Moves All Over the Sound Stage
I have a beautiful digital recording of Isaac Stern and the NY Philharmonic playing the Beethoven Violin Concerto in D Major. The solo violin sounds sometimes in the center, sometimes from the left speaker and sometimes like two different violins in two different locations. What is wrong?