Verify the Pinch roller has a barrel shape vs. flat surface. The MS-16 used a flat PR which then was updated to a slightly barrel shaped in later machines and the ATR-60. Both machines addressed the tape shifting with the dreaded 'Fine tape path alignment" in the manual. Put a good tape on, have both reels have the same amount of tape, and rock the tape with the FFD and RWD buttons and watch the tape as it changes directions. You might be able to dial it in with the tension roller height adjustment.