You mentioned earlier that the lights dim win the AC compressor kicks on. It still sounds like a voltage drop issue. If you can and have a voltage recorder of a max/min setting on your meter, I'd track the voltage as close as possible to the light.
Can you guesstimate the wire size of the utility's triplex? I remembered doing a service upgrade one time and the old service had #6 or possibly #8. It was and old 60 amp service put in a 200 amp service. The poco showed up and just reconnected the existing service drop
?=^P
