I agree with last poster that it is time to rewire, but it sound like you have marginal circuit sizing and under size wire. I suggest you increase the number of circuits and increase the wire size for voltage drop.
This sames like are there just too many lights on a circuit. While circuit is not overloaded the it probably is pushing the 80% load. You need to get the load down so you are pulling no more than 80% including the 125% for continuous duty.
