[mactec54]

Possibly:

Induced noise from something.
Ground all motor cases to the ground wire.

You can try it with a separate, testing only, wire from each motors case to gnd.
This happens with some AC brushless servos (with differential signals, to csmio industrial controller).

Perfectly repeatable, and goes away immediately via grounding the case.

This was the first thing I suggested, motor Grounding is important, these motors have a Ground wire attached inside the motor case, but he needs to check all his Grounds with a meter to make sure he has a good Ground contact, from the motor case to the end of his Ground wire