I had a similar problem with my 1985 300sd. It turned out to be the alternator. I took it to SEARS to have the battery tested thinking that it was the battery. They told me that I needed a new alternator and that I would have to leave the car overnight a pick it up the next day. Said it would cost 326 dollars. There was no way I was going to leave my car and let SEARS do any work on my car besides changing the tires! So I drove the car to my mechanic and he too concluded that the problem was in the alternator. The difference is instead of telling me I needed a new alternator for 250 or so dollars, he just replaced the alternator brushes and I was as good as new. And the brushes only cost 50 usd, so I ended up only spending about 125 usd for the job.