1992 Bonneville headlight problem

Good Day, Own a 1992 Pontiac Bonneville with a rebuilt starter, alternator. It also has a one year old battery. Problem I am having when I acclerate the headlights get brighter and when I lay off the gas the headlights go dimmer. I just checked the battery and the alternator and both check out OK. The battery terminals are clean, if that matters. Any suggestions? Thanks- :)

Voltage fluctuations. Alternator speed is proportional to voltage output. do you have a voltmeter on the instrument panel? Does voltage increase & decrease with engine speed? Maybe voltage regulator malfunction.