The upper and lower shafts of a sewing machine must work perfectly in parallel or the balance of the machine becomes unstable. If the handwheel gets bumped, the upper shaft can easily be jolted out of alignment. This is an easy fix. On the upper shaft, looking right to left; notice the bracket that separates the cam stack area from the needle bar assembly. A collar appears just before the needle bar bracket. Loosen the set screw. Bump or pull the shaft until it balances with the lower shaft, and retighten.