Power dip? Have you tried a different lamp? Is there a capacitor? If so perhaps it's failing? I'd start with checking the voltage (or plug in another street light on the same circuit and see if it also behaves the same way. If voltage is good or the other light (try to make it same ballast type but wattage or lamp type don't matter) doesn't flicker than it could be the lamp, dying capacitor, or maybe a poor neutral connection inside the fixture.