Every since I've owned the car (1991 300E), I've had this problem. Obviously only noticeable at night, but when the car is revving past 5000rpm....the headlights and dash lights dim quite a lot. After it shifts, everything comes back bright again. Just feels "ghetto".....
Car had a newer white Benz battery when I got the car, and now has a Red top Optima. The drive belt is tensioned properly. I do have Euro headlights, upgraded stereo, etc..... but even at idle, with all the accessories on, it doesn't have a problem powering everything. Just the noticeable dimming at very high RPM. I threw a new VR/Brushes at it shortly after purchasing it.