Line to neutral voltage with different 120 volt loads on? There could be a faulty neutral connection at the transformer or downstream to the meter even.
Type of lamps? If they are from a big box store, the quality could be suspect.
Multi-wire circuit? A bad neutral connection between the panel and the end of the multiwire circuit (three wire cable using two hots and one neutral) could cause temporary higher voltage on one leg of the circuit.
Are these fixtures located in the first floor ceiling of a two story house? The vibration from walking, high heel shoes, kids running etc. on the upper level of the house can be very hard on incandescent filaments and on compact fluorescent lamps.
Last resort, if the homeowner is very frustrated, rent a power quality monitor for a month and install on one of the circuits in question.
Good luck!
Larry
