E55 Electrical problem

I have a slight power inconsistency and not sure where to start the diagnosis. When driving at normal speed and then coming to a stop at a red light with normal breaking, the engine seems to take on an instantaneous alternator load slowing the engine speed, and the headlights slightly dim. And then within a split-second all is back to normal. The fluctuation is almost imperceptible. Any ideas, I would appreciate. JB