How quickly does the brightness of the lights change? If they flicker really fast, start by cleaning and tightening the battery cable connections and where the smaller positive battery wire bolts to the under-hood fuse box. If there's a large bolted-in fuse, tighten him too. If the brightness changes every few seconds or minutes, suspect the generator. You would find that by measuring the battery voltage with the engine running. Watch for a change in voltage that corresponds to a change in light brightness. If the voltage drops to less than 13.75 to 14.75 volts, or goes higher than that range, suspect the voltage regulator inside the generator. Due to the design of these generators, they develop huge voltage spikes that destroy the internal diodes and voltage regulator, although those problems aren't usually intermittent. It is common to go through four to six replacement generators in the life of the vehicle. Replacing the battery, which you already did, is what it takes to reduce the number of repeat failures, unless that battery is less than about two years old. As they age they lose their ability to dampen and absorb those harmful voltage spikes.