agree with above post. My 92 300e had same problem due to bad V regulator.

In my case, I checked battery voltage (after charging) 12v and then started the engine re-checking battery voltage which should now be 13.5 to 14 engine running.

In my case with engine running battery v stayed at 12. I gently tapped the voltage regulator (at rear bottom of alternator) and the engine idle changed slightly, this also produced a slight whining noise from the alternator.

Now batt v was at 14v. and I was able to continue to drive the vehicle until I replaced the regulator. I connected a multimeter to the cigarette lighter to keep an eye on the charging system. I only had to tap the regulator (occasionally) one time after start up.

On the off chance that yours is a similar problem, this may help.
good luck ,
Acky