I had a similar problem with my 93 300E 2.8. It turned out to be the alternator. Driving along the engine would run rough, then get well, then all the lights in the dash came on a few miles before the battery was too weak to operate the car. Replaced alternator (had it rebuilt about a month before the trouble.) Prior to that it would let battery die, and all the lights would come on occasionally and then get well. After the rebuild, the battery was staying charged, but still got the craziness with the lights. All this happened after I replaced a weak battery that wouldn't hold a charge. Maybe it killed the alternator by requiring charge all the time. Final cure was a new Bosch alternator. No problems since. Take it to a good tech with the right diagostic tools and they can confirm the problem before you buy one. They aren't cheap!