There could be a lot of contributing factors, POCO loads on the transmission lines, Vd in the customer owned service feeder and sub-feeders, and the branch circuit.
Vd calcs for the above for starters. Thorough recording of events (Voltage and amperage) at main and sub panel(s) will give more details. You have 500' +/- of conductors. Also, the AC unit may be starting to have issues.
One side ? for my curious mind....AC on at this time of year?
The amount of time that would be required to do what I suggest, vs. the swap to 230/240 volt fixture may take care of the HPS light issue.
Good Luck
