Is this normal...or is my alternator getting weak?
 

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".....:D

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.

Could be your voltage regulator on the back of the alternator. Mine needs changing too. Its around $30-$35 for a new one.

Well first check your battery voltage readings with the car off and on, then I suggest doing as 86560SEL suggested and replace the Voltage Regulator. 2 screws and it's in your hand, literally. The brushes wear out over time and can cause issues like this. Throw the one you remove into the trunk and forget about it. It's one of those "in case" items you should have in the trunk in case you are stuck in the middle of nowhere 5 years from now and your VR is toast...Might just get you home. I got an VR, OVP, Fuel pump relay and all fluids in the trunk of both of my cars. You never know!

;)

The old brushes were fairly worn, but the new ones didn't seem to help the problem much.

Next time a see a nice looking alternator at the junkyards, I will grab it.

Sounds like it blew a coil or a diode...... Yeah, I'd replace the alternator next, but I'd double check all my connections first.