Mechanic Q: Headlights change intensity?

Headlights flicker/change intensity at certain rpm. With ac on very noticeable. Without it is slight. Not consistent. Usually around 1500 rpm. From idle rev to 1500 slowly get a flicker/dimming then bright. 4000 miles on car two dealers say "everything in spec" no problem. This car have an electrical problem? What should we do?