Electrical issue with a 1991 300se
 

I have the car listed above and was driving it home last night when the lights dimmed and it lost nearly all electrical power. When I got home, I shut it off and it would not start. It seemed like a dead battery. This morning, it started right up and ran for about 5 minutes before losing electrical power again, lights dimming, etc. The motor continued to run fine but the tach was acting wildly. I shut it off, and it would not start. 5 minutes later it started again with low electrical power. Any ideas what this is?

Sounds exactly like a charging problem