Given that you are having issues with lights on a certain circuit, unless you just by chace got a bad batch of bulbs, I would start looking at the circuit. Maybe there is loose conncetion some where or it is somehow getting spiked. I would be looking at:
What is different about the questioned circuit compared to the other circuits that are similar?
What is the socket voltage on the troulbled circuit comepared to the other sockets one the good sockets?
What is the line voltage at the first and last light on a goood and bad circuit and the time of the day?
How is the power quality to the site?
If the neutrals is shared, what are they shared with?
Is the neutral properly shared all the way back to the panel?
