Need to check 240vac on dryer or range outlets, or directly in breaker panel. Then check each L side to neutral for balance which would indicate a neutral connection issue.
Neutral for overhead lines is the uninsulated support cable so it is carrying the mechanical stress of supporting the lines as well as neutral current. This support cable has a neutral connection clamp that is exposed to the weather.
Power companies have to predict load on grid and stoke the fire to boil more water to push the generators. This takes several hours to adjust. Load on network varies a lot during the day and if power company doesn't make the correct prediction of demand the voltage will rise or fall from normal average.
Reaction time depends on utility sources. Nuclear power plants are slowest to react, natural gas turbines are quickest.
My grid usually starts to rise about 8-9 pm as usage starts to lighten on grid. I occasionally see 250vac peak and once in a while it peaks close to 255 vac. In the mornings I am usually dead on 240vac.
A few months ago, after never happening in over twenty years of service on my house, my inverters started to drop the grid due to AC input overvoltage at 256 vac. I finally had to bump up the max AC input setting a couple of volts to keep inverters from disconnecting from grid.
Other side of extreme is being at the 'end of the line'. As your area develops, sometimes the utility just band-aids on extensions to accommodate new residential developments resulting in too much voltage drop. Some of their HV transformers have several taps to adjust voltage delivered.
