[machintek]

Lets look at a few things in a logical manner.
Since it will change tools, the spindle encoder functions in that it achieves orient and I am assuming that spindle speed is being achieved correctly. It also means that the spindle drive balance is set close enough. Otherwise it would not finish changing a tool.
I know there is a encoder in the spindle motor as well. It may this one that is giving you grief.
Have you tried this with the electrical cabinet door open and watched the spindle drive for a fault display? The undervoltage fault occurs when the drives shut off. Do not be concerned with that one.
I have had only one failure of this encoder in the spindle motor.
Have you tried tapping without evoking rigid tapping? Just to see what would happen?
Is this the first time you have tried rigid tapping? I know on some machines it was a option. We would call Bridgeport, give them the serial number of the DX32 control and they would give us a cryptic code to put in to enable it. Just thinking out loud. It has been a while since I played with this.
In Maintenance page watch the s axis error and run a rigid tap cycle. The error should be a degree or less (if I remember correctly). That may shed some light on it too.

George