Doesnt sound normal to me, check your wiring to make sure there is no breakdown in the isolation of the wires.
If not then could well be internal insulation that is breaking down. Not sure if it is safe to test a stepper with a megger or not, dont see why not but wouldnt like to say for sure. If it is safe and you did get one to test then that would tell you if the insulation in the motor is breaking down.
Hood