This looks ‘rough’, for a good 3 phase supply??

Or, it may be the resolution of the recorder, 

I will have a ‘think, and come back to you.

With the history of failure I believe the supply must be the problem, from what you have shown this looks to be ok with regard to Harmonic distortion.

The environment is worth considering, is it clean and dust free, are the components enclosed within a secure enclosure, with adequate ventilation?

These are the things which quickly become evident from a site visit, but this is not possible, so I can only ask perhaps, silly questions, hoping something will become clear.

Sam, Come back if I can help further..

Thanks for the feedback, does this mean that you have replaced the drive unit? If all is working well now you will be leaving this to run, hoping for no further trouble??

As with many issues like this, it is not always possible to determine what the exact fault, and it may be prudent to get on to more ‘pressing’ matters for the time being.

Best of luck.