ACC-51P Discontinuous Jump in Position

System: air bearing spindle with brushless rotary motor, 9000 line sin-cos encoder, ACC-51P. Was working fine and then after making some software changes to a PLC suddenly behaves very strangely in closed loop, even after doing $$$*** reset, power cycling, etc. Open loop the system is quiet, position noise is only an LSB or 2, closed loop the following error toggles close to 64 full counts (2048 LSB's), mostly in one direction. It ran away once. It is like something is stepping on either the DAC or the encoder channel. The system moves smoothly doing open loop command with modest currents, rotates endlessly in both directions. It doesn't appear to lose any counts when rotated.

 

Have moved the motor to different DAC's, same result.

 

Anyone know what failure mode or noise would cause this?

Here are some pictures, one with the system open loop with the drive disabled, two in closed loop at different scales. Frequency is just over 100 Hz, but definitely not 120 Hz according to FFT. Jumps appear to be 64 counts or 32 counts. When quiet, the system is still to a few LSB's (subcount).

On further analysis, this looks like 120 Hz rectifier noise. I am not sure how it suddenly appeared out of no where, but perhaps we lost a ground somewhere or busted a component.