Generally, utilities reference a national standard that limits sustained voltage at the delivery point (meter) to 126/252 or maybe 127/254. but it's usually voluntary unless enforced by a state Public Utilities Commission. If the utility won't act on your complaint, then the PUC might.
It sounds like you have enough evidence for a valid complaint. Be aware that temporary voltage excursions are harder to control than if sustained.
If you are going to quote numbers, make absolutely sure you have a known-accurate voltmeter. 115-120 volts is ideal. Their transformer may or may not have taps for voltage adjustment. Be aware of the accuracy extremes of the meter, and allow for the possibility of errors in readings. It is possible that the PUC may do independent checks.
In the US, many utilities and state PUCs have websites that may contain specifics on customer-service conditions, but may not be easy to find.
One solution on your own may be to have an electrical contractor install "buck" transformers, to reduce voltage by, say, 12 volts in 120. Buck transformers do not regulate anything, but reduce voltage to the load as a fixed percentage of the source.
{The low end is about 110/220, or 106/212 for other than incandescent lighting.}
--s falke
"Haskell Lee" wrote...