the problem with overriding the traverse speed.
The only possible reason I can see is the problem with the power supply.
I have already such experience, that if the load on the supplied power transformer
suddenly change, the power factor is overcompensate, the value which
normally is approx. 0.96, goes to value bigger than 1.
If the power factor is over compensated, the AC motor is without control.
The torque and motor speed reach its maximum values, motor is not controlled.
I suggest to record the power supply parameters, especially the power factor,
and in the same time observe the traverse speed.
Attached information about consequence of power factor over compensation.