You don't mention your mileage, but if it's "up there", I'd check the brushes on the alternator for wear. I believe it's an assembly ( VR module ) which can be removed and changed fairly easily. Along the same line, a loose alternator belt could be slipping. I have a 1991 300SL and found I had a badly corroded cable at the positive battery terminal. The corrosion was under the black plastic insulation, so it wasn't apparent to visual inspection. Tod
Resistance is Futile.