You need some data logging to help you identify what's going really on.
Do you have a UPS that reports voltage states?
An Alarm panel with Cell Backup that can help you know if you are just loosing comms or if power is the issue?

Did the PoCo hookup a logger and let it run for a week or just test at the meter and declare it was ok in the 15 minutes they were there.

Does the site have generator with an automatic transfer switch?