It would point to a phase loss, but does not have to be incoming power. The last situation we had was a piece of wiring insulation was not stripped all the way back on a new drive to the contactor. The resulting phase loss, opened a coil on the motor and the drive started faulting out. It took a good two days to find it. Of course it seems you would have eliminated all of this by replacing the wye delta with soft stater.