When there is a voltage reduction every few minutes or so, its like turning the light bulbs on and off. The on and off turning is what causes the filament to degrade and the life expectacy is decreased.
That is exactly what does not happen. Power cycling does not harm any incandescent bulb. Again, summarized from the industry 'bible' are the only things that reduce bulb life expectancy.
Bulbs changing intensity with a 1000 watt load suggests a wiring problem. Critical is to identify intensity changes on that circuit AND on any other circuit. Since in most cases, that wiring problem is only a nuisance. But in some rare cases, it is a serious human safety issue.
AC voltage is 120 volts. If voltage is constantly exceeding 127 volts, then bulbs are brighter and last only half as long. But again, what is changing intensity and how (dimmer or brighter) are critically important facts. Do not make conclusions based in speculation. You do not know yet what is causing bulb failure. Dimming would only increase bulb life expectancy (irrelevant if by a lot or a little). Dimming (and power cycling) does not shorten the life expectancy of any incandescent bulb.
Also discussed was damage due to vibration - such as someone walking across the upstairs floor.