My customer is shipping the unit back to me. As soon as I receive it, I'll be able to get more detailed info. To answer some of the questions and give an update:
When the problem first appeared, when it goes black, the PLC seemed to operate normally until the very last process. The screen would go black sporadically during the process, was not consistent with any particular event. As time went on, it went black more frequently. To return it to life, we had to switch off/on power to the unit. However I was just informed that the PLC is no longer working. Instead of troubleshooting, we are purchasing a replacement (this unit was purchased over 3 years ago), as it's a time-sensitive issue and can't hold up production. As I've mentioned, it's the first time one has failed like this.