Sounds like a sporadic alternator telling you it's time to replace it...I could be wrong but if your alternator's never been changed and it's a '93 or older car, I'd almost consider an alternator to be a maintenance item.
Don't know what year your car is, but 300Es had sorta weak alternators and it's very little trouble upgrading...I know it's been discussed on this Forum. Had I known I'd have installed a larger alternator in my wagon, as a 275 watt system is going in there soon (and I may need to install a capacitor, since my alt. only puts out 65 watts or so; I suggest you check your owner's manual).
Best of luck
Best regards, Michael