You need to monitor voltage to see what it is. Hobo might make a cheap
voltage recorder to hook to your computer. Without knowing what the drop
and duration are you will have trouble finding it. Even now having a
volt meter where you can see a drops voltage would help. You might
contact an electric supply store they might stock a meter that can store
data. Google Voltage Recorder. Just because the electric company did not
see their equipment as bad, it could be.