Spindle motor phase variance

We have a St-40N that has been shorting out our spindle drive. We sent the drives off to Mitsubishi and they came back saying we had a power semiconductor failure. After talking with Mazak they recommended checking for a variance in ohms between the phases of the motor. We megged the motor and it checked fine. Mitsubishi then told us that some other machines had the same problem that they megged fine but we needed to use a milliohm meter to check for slight variations between the phases.
Before we drop $600 for a milliohm meter that we will probably only use this once, I was wondering if anyone here has experienced the same problem and if you did what was needed to correct it?

The alarms we had gotten were 21- drive power malfunction; 204 Spindle control malfunction; 33 emergency stop , on the control.
The alarms on the drive itself was F6-32

Any help would be much appreciated.

Did you also check for winding short to motor case?
You can rent a meter instead of buying it.